tests: introduce test_must_fail

When we expect a git command to notice and signal errors, we
carelessly wrote in our tests:

    test_expect_success 'reject bogus request' '
        do something &&
        do something else &&
        ! git command
    '

but a non-zero exit could come from the "git command" segfaulting.

A new helper function "tset_must_fail" is introduced and it is
meant to be used to make sure the command gracefully fails (iow,
dying and exiting with non zero status is counted as a failure
to "gracefully fail").  The above example should be written as:

    test_expect_success 'reject bogus request' '
        do something &&
        do something else &&
        test_must_fail git command
    '

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/t/t2008-checkout-subdir.sh b/t/t2008-checkout-subdir.sh
index 4a723dc0e..3e098ab31 100755
--- a/t/t2008-checkout-subdir.sh
+++ b/t/t2008-checkout-subdir.sh
@@ -68,15 +68,15 @@ test_expect_success 'checkout with simple prefix' '
 '
 
 test_expect_success 'relative path outside tree should fail' \
-	'! git checkout HEAD -- ../../Makefile'
+	'test_must_fail git checkout HEAD -- ../../Makefile'
 
 test_expect_success 'incorrect relative path to file should fail (1)' \
-	'! git checkout HEAD -- ../file0'
+	'test_must_fail git checkout HEAD -- ../file0'
 
 test_expect_success 'incorrect relative path should fail (2)' \
-	'( cd dir1 && ! git checkout HEAD -- ./file0 )'
+	'( cd dir1 && test_must_fail git checkout HEAD -- ./file0 )'
 
 test_expect_success 'incorrect relative path should fail (3)' \
-	'( cd dir1 && ! git checkout HEAD -- ../../file0 )'
+	'( cd dir1 && test_must_fail git checkout HEAD -- ../../file0 )'
 
 test_done