Some tests are skipped due to the "perf" filtering

[Alberto Donato]

The way we're currently filtering performance tests out with `--exclude=perf` in the runner command line is causing more tests than intended to be skipped.

AFAICT the exclude is matched against the full test name, so a test which contains the "perf" string in the name is skipped.
There are a few tests that contains strings like "performs" or "performed" in the name which are also skipped.

I think we should move the performance tests to a separate top-level package (possibly with different subpackages to match code structure) so that we can just skip/select the package in each runner.