Fix-up behavior for empty PerfGroup

[topolarity]

For machines that don't have perf properly configured, it's common to end up with an empty PerfGroup that has an invalid leader_fd. This commit adds the appropriate guards to make sure that doesn't error excessively.

On my WSL2 setup, this takes the test suite from:

Test Summary:      | Pass  Error  Total  Time
LinuxPerf          |   31      4     35  4.6s
  simple benchmark |           2      2  2.6s
  @measure         |           1      1  0.1s
  Parser           |   24            24  0.3s
  @pstats          |    1      1      2  1.5s
  _addcommas       |    6             6  0.0s
ERROR: LoadError: Some tests did not pass: 31 passed, 0 failed, 4 errored, 0 broken.

to:

Test Summary: | Pass  Total  Time
LinuxPerf     |   37     37  3.1s

[Zentrik]

I thought perf didn't work at all on wsl2?

[topolarity]

WSL2 has better support than WSL1 (depending on your HW/kernel configuration), but it's missing many of the timers you might expect.

That's what this is fixing - when none of the timers are supported the EventGroup can end up empty and then this behaves poorly.

[Zentrik]

Perhaps we should warn after constructing an EventGroup if its leader_fd is -1 or if any events within it are disabled. Looks like we already do.