refactor mechanism for testing running of 'spring test'

This will help with the next commits.
make "spring test" support multiple arguments

"spring test" needs to accept multiple arguments if guard-minitest is
to ever have a chance of supporting spring.
make "spring test" run all unit tests when given no arguments

With no arguments, it now defaults to running all test-unit tests it
can find, i.e. files matching test/**/*_test.rb.  This is more useful
and arguably more intuitive too.

We need to remove test/performance/browsing_test.rb from the Rails app
being tested, otherwise "spring test" attempts to run this when it
iterates over test/**/*_test.rb, causing a failure due to ruby-prof
not being installed.