t: add a function to check executable bit

In line with our other helper functions for paths, let's add a function
to check whether a path is executable, and if not, print a suitable
error message.  Document this function, and note that it must only be
used under the POSIXPERM prerequisite, since it doesn't otherwise work
on Windows.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -901,6 +901,15 @@ test_path_is_symlink () {
 	fi
 }
 
+test_path_is_executable () {
+	test "$#" -ne 1 && BUG "1 param"
+	if ! test -x "$1"
+	then
+		echo "$1 is not executable"
+		false
+	fi
+}
+
 # Check if the directory exists and is empty as expected, barf otherwise.
 test_dir_is_empty () {
 	test "$#" -ne 1 && BUG "1 param"