ts: fix is_modules_defined test

Add bash script to test whether module function is defined

Author: xdelaruelle

testsuite/is_module_defined

@@ -0,0 +1,2 @@
+#!/bin/bash
+type module >/dev/null 2>&1

testsuite/modules.00-init/005-init_ts.exp

@@ -681,7 +681,7 @@ unset orig_perms
 unset new_perms
 
 # check if module command is defined prior running tests
-set is_modules_defined [info exists ::env(MODULES_CMD)]
+set is_modules_defined [expr {![catch {exec $env(TESTSUITEDIR)/is_module_defined}]}]
 
 # check if symbolic link registered in git repository have been effectively
 # created as symbolic link to determine if they are supported on filesystem