t7110: replace test -f with test_path_is_* helpers

`test -f` and `! test -f` do not provide clear error messages when they fail.
To enhance debuggability, use `test_path_is_file` and `test_path_is_missing`,
which instead provide more informative error messages.

Note that `! test -f` checks if a path is not a file, while
`test_path_is_missing` verifies that a path does not exist. In this specific
case the tests are meant to check the absence of the path, making
`test_path_is_missing` a valid replacement.

Signed-off-by: Matteo Bagnolini <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

t/t7110-reset-merge.sh

@@ -270,22 +270,22 @@ test_expect_success '--merge is ok with added/deleted merge' '
 	git reset --hard third &&
 	rm -f file2 &&
 	test_must_fail git merge branch3 &&
-	! test -f file2 &&
-	test -f file3 &&
+	test_path_is_missing file2 &&
+	test_path_is_file file3 &&
 	git diff --exit-code file3 &&
 	git diff --exit-code branch3 file3 &&
 	git reset --merge HEAD &&
-	! test -f file3 &&
-	! test -f file2 &&
+	test_path_is_missing file3 &&
+	test_path_is_missing file2 &&
 	git diff --exit-code --cached
 '
 
 test_expect_success '--keep fails with added/deleted merge' '
 	git reset --hard third &&
 	rm -f file2 &&
 	test_must_fail git merge branch3 &&
-	! test -f file2 &&
-	test -f file3 &&
+	test_path_is_missing file2 &&
+	test_path_is_file file3 &&
 	git diff --exit-code file3 &&
 	git diff --exit-code branch3 file3 &&
 	test_must_fail git reset --keep HEAD 2>err.log &&