test_suite: Use the portable find -perm -111 to find executables.

find -executable is only available on GNU. find -perm -111 finds
all executable files by their mode (although it won't find
executable files in access control lists on Linux, which
isn't a problem in this case)

spec/test_app/test_suite.sh

@@ -2,5 +2,4 @@
 
 cd $(dirname $0)
 
-find scripts -type f -executable -exec '{}' \;
-
+find scripts -type f -perm -111 -exec bash '{}' \;