tests: no --only minimum coverage requirement. (#299)

This will almost certainly fail every time so let's disable this error.

Library/Homebrew/cmd/tests.rb

@@ -30,6 +30,7 @@ module Homebrew
       args = []
       args << "--trace" if ARGV.include? "--trace"
       if ARGV.value("only")
+        ENV["HOMEBREW_TESTS_ONLY"] = "1"
         test_name, test_method = ARGV.value("only").split("/", 2)
         args << "TEST=test_#{test_name}.rb"
         args << "TESTOPTS=--name=test_#{test_method}" if test_method

Library/Homebrew/test/.simplecov

@@ -3,7 +3,7 @@
 SimpleCov.start do
   tests_path = File.dirname(__FILE__)
 
-  minimum_coverage 40
+  minimum_coverage 40 unless ENV["HOMEBREW_TESTS_ONLY"]
   coverage_dir File.expand_path("#{tests_path}/coverage")
   root File.expand_path("#{tests_path}/../../")