ci(github): avoid printing test case preamble twice

We want to mark up the test case preamble when presenting test output in
Git's GitHub workflow. Let's suppress the non-marked-up version in that
case. Any information it would contain is included in the marked-up
variant already.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib-github-workflow-markup.sh b/t/test-lib-github-workflow-markup.sh
index 1ef0fd5..9c5339c 100644
--- a/t/test-lib-github-workflow-markup.sh
+++ b/t/test-lib-github-workflow-markup.sh
@@ -20,6 +20,8 @@
 # workflows; these functions will then override (empty) functions
 # that are are called at the appropriate times during the test runs.
 
+test_skip_test_preamble=t
+
 start_test_output () {
 	test -n "$GIT_TEST_TEE_OUTPUT_FILE" ||
 	die "--github-workflow-markup requires --verbose-log"