[PATCH 2/2] test-lib-functions: use BUG() in 'test_must_fail'

["SZEDER Gábor" <szeder.dev@gmail.com>]

In many test helper functions we verify that they were invoked with
sensible parameters, and call BUG() to abort the test script when the
parameters are buggy.  6a67c75948 (test-lib-functions: restrict
test_must_fail usage, 2020-07-07) added such a parameter verification
to 'test_must_fail', but it didn't report the error with BUG(), like
we usually do.

As discussed in detail in the previous patch, BUG() didn't really work
in 'test_must_fail' back then, but it resolved those issues, so let's
use BUG() in this case as well.

The two tests checking that 'test_must_fail' recognizes invalid
parameters need some updates:

  - BUG() calls 'exit 1' to abort the test script, but we don't want
    that to happen while testing 'test_must_fail' itself, so in those
    tests we must invoke that function in a subshell.
  - These tests check that 'test_must_fail' failed with the
    appropriate error message, but BUG() sends its error message to a
    different file descriptor, so update the redirection accordingly.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t0000-basic.sh        | 4 ++--
 t/test-lib-functions.sh | 3 +--
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index a6e570d674..b9d5c6c404 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -1315,12 +1315,12 @@ test_expect_success 'test_must_fail on a failing git command with env' '
 '
 
 test_expect_success 'test_must_fail rejects a non-git command' '
-	! test_must_fail grep ^$ notafile 2>err &&
+	! ( test_must_fail grep ^$ notafile ) 7>err &&
 	grep -F "test_must_fail: only '"'"'git'"'"' is allowed" err
 '
 
 test_expect_success 'test_must_fail rejects a non-git command with env' '
-	! test_must_fail env var1=a var2=b grep ^$ notafile 2>err &&
+	! ( test_must_fail env var1=a var2=b grep ^$ notafile ) 7>err &&
 	grep -F "test_must_fail: only '"'"'git'"'"' is allowed" err
 '
 
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index a40c1c5d83..cdbc59e4f0 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -910,8 +910,7 @@ test_must_fail () {
 	esac
 	if ! test_must_fail_acceptable "$@"
 	then
-		echo >&6 "test_must_fail: only 'git' is allowed: $*"
-		return 1
+		BUG "test_must_fail: only 'git' is allowed: $*"
 	fi
 	"$@" 2>&6
 	exit_code=$?
-- 
2.30.1.940.gce394404de