TESTS: make time consuming resampler tests run as slow tests

[swesterfeld]

I made every resampler test that is somewhat slow run as slow test. This reduces the time it takes to execute all resampler tests on my machine from 1.9 seconds to 0.3 seconds if only one cpu is used. Note that although I added

  if (Bse::Test::slow())

to execute the resamplehandle benchmarks if and only if we're running as slow tests, I found that the resamplehandle benchmarks are now never executed (which probably doesn't matter). I tried to investigate, Bse::Test::slow() does something like

  static bool cached_slow = feature_toggle_bool (getenv ("BSE_TEST"), "slow");

but that variable is never set elsewhere in the code (or at least I don't find where it is).