tests: use 'test_must_be_empty' instead of '! test -s'

diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
index 0c61268fd22ade66c6cbd91ad743986b33af0d5d..9e88a56d619ca33a24089ec1208e6726d76f6256 100755 (executable)
--- a/t/t0090-cache-tree.sh
+++ b/t/t0090-cache-tree.sh
@@ -239,7 +239,7 @@ test_expect_success 'no phantom error when switching trees' '
        >newdir/one &&
        git add newdir/one &&
        git checkout 2>errors &&
-       ! test -s errors
+       test_must_be_empty errors
 '
 
 test_expect_success 'switching trees does not invalidate shared index' '