leak tests: fix a memory leak in "test-progress" helper

Fix a memory leak in the test-progress helper, and mark the
corresponding "t0500-progress-display.sh" test as being leak-free
under SANITIZE=leak. This fixes a leak added in 2bb74b5 (Test the
progress display, 2019-09-16).

My 48f6871 (tr2: stop leaking "thread_name" memory, 2021-08-27)
had fixed another memory leak in this test (as it did some trace2
testing).

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/helper/test-progress.c

@@ -69,6 +69,7 @@ int cmd__progress(int argc, const char **argv)
 			die("invalid input: '%s'\n", line.buf);
 	}
 	stop_progress(&progress);
+	strbuf_release(&line);
 
 	return 0;
 }

t/t0500-progress-display.sh

@@ -2,6 +2,7 @@
 
 test_description='progress display'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 show_cr () {