cli: fix running multiple test files

[pushcx]

Running be qt test/1.test.rb test/2.test.rb silently ignored all files after the first one. Dir.glob accepts an array, so this is an easy fix.

I dislike the temporal coupling of parse_options and parse_args caused by OptionParser mutating its inputs, but I saw it as a separate issue.

[willcosgrove]

I think this is broken when you run qt with no arguments, because the @args array is empty but truthy, so the default value of "./**/*.test.rb" never gets used.