Hey Alexander,
Thanks for your answer.

In order that my changes work, Some patches on bitbake should be accepted:
https://lists.openembedded.org/g/bitbake-devel/message/14815?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2Cbelouarga%2C20%2C2%2C0%2C99163598
https://lists.openembedded.org/g/bitbake-devel/message/14819?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2Cbelouarga%2C20%2C2%2C0%2C99230772
https://lists.openembedded.org/g/bitbake-devel/message/14820?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2Cbelouarga%2C20%2C2%2C0%2C99248868

and one patch that I just made on open embedded-core:
https://lists.openembedded.org/g/openembedded-core/message/182022?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2Cbelouarga%2C20%2C2%2C0%2C99248914

I ran the tests that you asked me to do, you can find the results below.

elf@container:~/workspace$ oe-selftest -r recipetool.RecipetoolCreateTests.test_recipetool_create_npm
2023-05-31 17:31:33,552 - oe-selftest - INFO - Changing cwd to /home/elf/workspace/build
2023-05-31 17:31:33,552 - oe-selftest - WARNING - meta-selftest layer not found in BBLAYERS, adding it
2023-05-31 17:31:39,880 - oe-selftest - INFO - Adding layer libraries:
2023-05-31 17:31:39,880 - oe-selftest - INFO -  /home/elf/workspace/layers/poky/meta/lib
2023-05-31 17:31:39,880 - oe-selftest - INFO -  /home/elf/workspace/layers/poky/meta-yocto-bsp/lib
2023-05-31 17:31:39,880 - oe-selftest - INFO -  /home/elf/workspace/layers/meta-openembedded/meta-oe/lib
2023-05-31 17:31:39,880 - oe-selftest - INFO -  /home/elf/workspace/layers/poky/meta-selftest/lib
2023-05-31 17:31:39,882 - oe-selftest - INFO - Running bitbake -e to test the configuration is valid/parsable
NOTE: Starting bitbake server...
2023-05-31 17:31:49,176 - oe-selftest - INFO - Adding: "include selftest.inc" in /home/elf/workspace/build-st/conf/local.conf
2023-05-31 17:31:49,178 - oe-selftest - INFO - Adding: "include bblayers.inc" in bblayers.conf
2023-05-31 17:31:49,179 - oe-selftest - INFO - test_recipetool_create_npm (recipetool.RecipetoolCreateTests)
2023-05-31 17:33:36,312 - oe-selftest - INFO -  ... ok
2023-05-31 17:33:38,097 - oe-selftest - INFO - ----------------------------------------------------------------------
2023-05-31 17:33:38,097 - oe-selftest - INFO - Ran 1 test in 115.852s
2023-05-31 17:33:38,097 - oe-selftest - INFO - OK
2023-05-31 17:33:48,971 - oe-selftest - INFO - RESULTS:
2023-05-31 17:33:48,971 - oe-selftest - INFO - RESULTS - recipetool.RecipetoolCreateTests.test_recipetool_create_npm: PASSED (107.13s)
2023-05-31 17:33:49,063 - oe-selftest - INFO - SUMMARY:
2023-05-31 17:33:49,063 - oe-selftest - INFO - oe-selftest () - Ran 1 test in 115.853s
2023-05-31 17:33:49,064 - oe-selftest - INFO - oe-selftest - OK - All required tests passed (successes=1, skipped=0, failures=0, errors=0)
elf@container:~/workspace$ oe-selftest -r devtool.DevtoolAddTests.test_devtool_add_npm
2023-05-31 17:34:03,550 - oe-selftest - INFO - Changing cwd to /home/elf/workspace/build
2023-05-31 17:34:03,550 - oe-selftest - INFO - Adding layer libraries:
2023-05-31 17:34:03,550 - oe-selftest - INFO -  /home/elf/workspace/layers/poky/meta/lib
2023-05-31 17:34:03,551 - oe-selftest - INFO -  /home/elf/workspace/layers/poky/meta-yocto-bsp/lib
2023-05-31 17:34:03,551 - oe-selftest - INFO -  /home/elf/workspace/layers/meta-openembedded/meta-oe/lib
2023-05-31 17:34:03,551 - oe-selftest - INFO -  /home/elf/workspace/layers/poky/meta-selftest/lib
2023-05-31 17:34:03,552 - oe-selftest - INFO - Running bitbake -e to test the configuration is valid/parsable
NOTE: Starting bitbake server...
2023-05-31 17:34:12,581 - oe-selftest - INFO - Adding: "include selftest.inc" in /home/elf/workspace/build-st/conf/local.conf
2023-05-31 17:34:12,582 - oe-selftest - INFO - Adding: "include bblayers.inc" in bblayers.conf
2023-05-31 17:34:15,320 - oe-selftest - INFO - test_devtool_add_npm (devtool.DevtoolAddTests)
2023-05-31 17:38:02,622 - oe-selftest - INFO -  ... ok
2023-05-31 17:38:04,371 - oe-selftest - INFO - ----------------------------------------------------------------------
2023-05-31 17:38:04,371 - oe-selftest - INFO - Ran 1 test in 238.516s
2023-05-31 17:38:04,371 - oe-selftest - INFO - OK
2023-05-31 17:38:21,118 - oe-selftest - INFO - RESULTS:
2023-05-31 17:38:21,118 - oe-selftest - INFO - RESULTS - devtool.DevtoolAddTests.test_devtool_add_npm: PASSED (227.30s)
2023-05-31 17:38:21,200 - oe-selftest - INFO - SUMMARY:
2023-05-31 17:38:21,200 - oe-selftest - INFO - oe-selftest () - Ran 1 test in 238.517s
2023-05-31 17:38:21,201 - oe-selftest - INFO - oe-selftest - OK - All required tests passed (successes=1, skipped=0, failures=0, errors=0)