refactor(test-runner-core): remove unused configFilePath option and TestSessionGroup type.

[wjhsf]

The primary purpose of this PR is to fix #2982. I removed the logic that makes test file discovery relative to the specified configFilePath. Now the behavior is the same as when no configFilePath is specified, i.e. files are found relative to the working directory. After removing that logic, I realized the configFilePath option was no longer used anywhere, so I removed it completely. This is a breaking change for anyone using that option. However, given that the option doesn't really do anything, this is probably of minimal impact.

While familiarizing myself with the code base, I also noticed that the sessionGroups variable is never used. Removing that and propagating the change through the code base ultimately led to the removal of the unused TestSessionGroup type and a change to the TestSessionManager constructor signature. These are also technically breaking changes for anyone who imports @web/test-runner-core directly.

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