refactor: fix ambiguous regexp / assertion in one of the tests

[pvdb]

Running rake test on the master branch outputs the following Ruby syntax warning:

.../test/test_rake_rake_test_loader.rb:57:
warning: ambiguous `/`; wrap regexp in parentheses or add a space after `/` operator

Wrapping the assert_match() arguments in parentheses fixes the ambiguity, and - what's more - makes the assertion syntax consistent with similar assertions throughout the rake codebase #consistency_ftw 🎉

before:

$ ruby -w -c test/test_rake_rake_test_loader.rb
test/test_rake_rake_test_loader.rb:57: warning: ambiguous `/`; wrap regexp in parentheses or add a space after `/` operator
Syntax OK
$ _

after:

$ ruby -w -c test/test_rake_rake_test_loader.rb
Syntax OK
$ _

[hsbt]

👍 Thanks!