t7605: use test_path_is_file instead of test -f

Replace old-style 'test -f' path checks with the modern
test_path_is_file helper in the merge_c1_to_c2_cmds block.

The helper provides clearer failure messages and is the
established convention in Git's test suite.

Signed-off-by: Mansi <mansimaanu8627@gmail.com>

Author: MansiSingh17

t/t7605-merge-resolve.sh

@@ -34,9 +34,9 @@ merge_c1_to_c2_cmds='
 	test "$(git rev-parse c1)" = "$(git rev-parse HEAD^1)" &&
 	test "$(git rev-parse c2)" = "$(git rev-parse HEAD^2)" &&
 	git diff --exit-code &&
-	test -f c0.c &&
-	test -f c1.c &&
-	test -f c2.c &&
+	test_path_is_file c0.c &&
+	test_path_is_file c1.c &&
+	test_path_is_file c2.c &&
 	test 3 = $(git ls-tree -r HEAD | wc -l) &&
 	test 3 = $(git ls-files | wc -l)
 '