t7406: prefer test_* helper functions to test -[feds]

test -e, test -s, etc. do not provide nice error messages when we hit
test failures, so use the test_* helper functions from
test-lib-functions.sh.

Also, add test_path_exists() to test-lib-function.sh while at it, so
that we don't need to worry whether submodule/.git is a file or a
directory.  It currently is a file with contents of the form
   gitdir: ../.git/modules/submodule
but it could be changed in the future to be a directory; this test
only really cares that it exists.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7406-submodule-update.sh

@@ -174,7 +174,7 @@ test_expect_success 'submodule update does not fetch already present commits' '
 	  git submodule update > ../actual 2> ../actual.err
 	) &&
 	test_i18ncmp expected actual &&
-	! test -s actual.err
+	test_must_be_empty actual.err
 '
 
 test_expect_success 'submodule update should fail due to local changes' '
@@ -620,8 +620,8 @@ test_expect_success 'submodule update --init skips submodule with update=none' '
 	git clone super cloned &&
 	(cd cloned &&
 	 git submodule update --init &&
-	 test -e submodule/.git &&
-	 test_must_fail test -e none/.git
+	 test_path_exists submodule/.git &&
+	 test_path_is_missing none/.git
 	)
 '
 

t/test-lib-functions.sh

@@ -565,6 +565,14 @@ test_path_is_dir () {
 	fi
 }
 
+test_path_exists () {
+	if ! test -e "$1"
+	then
+		echo "Path $1 doesn't exist. $2"
+		false
+	fi
+}
+
 # Check if the directory exists and is empty as expected, barf otherwise.
 test_dir_is_empty () {
 	test_path_is_dir "$1" &&