[SDTEST-2714] Provide a way to run a subset of tests or run tests from a custom location

[anmarchenko]

What does this PR do?

Adds support for custom test file discovery patterns via a new --tests-location flag. Users can now specify custom glob patterns to override the default test file locations, enabling DDTest to work with non-standard project structures or to run only some tests.

Motivation

Sometimes tests could located in unexpected location and sometimes we need to parallelize a subset of tests - for example if we are only interested in system tests we could use --tests-location test/system/**/*_test.rb

Additional Notes

• The flag works for both RSpec and Minitest frameworks
• Default patterns remain unchanged: spec/**/*_spec.rb for RSpec, test/**/*_test.rb for Minitest
• The custom pattern is used for both test discovery (DiscoverTests) and test file discovery (DiscoverTestFiles)
• Environment variable DD_TEST_OPTIMIZATION_RUNNER_TESTS_LOCATION is also supported
• Documentation updated in README with usage examples
• Comprehensive test coverage added for both RSpec and Minitest with custom patterns

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