Add support for KNAPSACK_PRO_TEST_FILE_LIST environment variable to run explicitly listed tests

[ArturT]

This allows you to distribute exactly list of test files separated by a comma that you provide to KNAPSACK_PRO_TEST_FILE_LIST.

For instance, you can run RSpec tests files or only some test or test group from inside of spec file if you provide line number.

KNAPSACK_PRO_TEST_FILE_LIST=spec/a_spec.rb:10,spec/b_spec.rb,spec/c_spec.rb:23,spec/c_spec.rb:40

As you can see you can provide a few lines inside of the same spec file as we did for spec/c_spec.rb:23,spec/c_spec.rb:40.

KNAPSACK_PRO_TEST_FILE_LIST can be useful when you have your own custom script that generates a very specific list of test file paths you want to run and it's not possible to configure it with KNAPSACK_PRO_TEST_FILE_PATTERN and KNAPSACK_PRO_TEST_FILE_EXCLUDE_PATTERN.

Another case is when you want to use KNAPSACK_PRO_TEST_FILE_LIST to run only some of the tests from spec file and you want to provide line number like KNAPSACK_PRO_TEST_FILE_LIST=spec/c_spec.rb:23,spec/c_spec.rb:40.