chore: use regex for checking undefined fuction warning

Author: tsloughter

shelltests/check_undefined_test/check_undefined_test.test

@@ -1,12 +1,3 @@
 $ TERM=dumb rebar3 release
->
-===> Verifying dependencies...
-===> Analyzing applications...
-===> Compiling check_undefined_test
-===> Assembling release check_undefined_test-0.1.0...
-===> There are missing function calls in the release.
-===> Make sure all applications needed at runtime are included in the release.
-===> check_undefined_test_app:start/2 calls undefined function crypto:start/0
-===> check_undefined_test_app:start/2 calls undefined function notamodule:dosomething/0
-===> Release successfully assembled: _build/default/rel/check_undefined_test
+> / calls undefined function notamodule /
 >= 0