Disable parallel for small test coverage runs

[samford]

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brew tests --coverage can fail to produce coverage information when run on a small number of tests (e.g., --only utils/curl). We use ParallelTests::last_process? in tests/spec_helper.rb to handle the SimpleCov output but due to the way the method is implemented, it doesn't work as expected if the number of processes is greater than one but lower than the number of cores.

For example, this is the current output for a small run (no coverage generated):

$ brew tests --only utils/curl --coverage
Randomized with seed 26057
1 process for 1 spec, ~ 1 spec per process
.........................................



Took 0 seconds

This is the output with this change (coverage generated as expected):

$ brew tests --only utils/curl --coverage
Randomized with seed 9280
Run options: exclude {:needs_linux=>true, :needs_network=>true, :needs_ci=>true, :needs_svn=>true}

Randomized with seed 9280
.........................................

Finished in 0.10351 seconds (files took 0.56166 seconds to load)
41 examples, 0 failures

Randomized with seed 9280

Coverage report generated for brew:80041 to /opt/homebrew/Library/Homebrew/test/coverage.
Line Coverage: 17.05% (8454 / 49580)
Branch Coverage: 2.32% (204 / 8797)
Coverage report generated for brew:80041 to /opt/homebrew/Library/Homebrew/test/coverage/coverage.xml. 8454 / 49580 LOC (17.05%) covered; 204 / 8797 BC (2.32%) covere

I tried to address this through changes to spec_helper.rb and/or changes to ParallelTests but didn't meet with any success.

This works around the issue by disabling parallel test execution when the --coverage option is used and the number of files to be tested is lower than the number of CPU cores. I've been using this workaround for months and it works as expected on my machine (M1 Pro and now M4 Pro).

[MikeMcQuaid]

Thanks for explanation and PR! Have a proposed, simpler change.

[MikeMcQuaid]

Suggested change

	parallel = false if args.profile || (args.coverage? && files.length < Hardware::CPU.cores)
	parallel = false if args.profile || args.only

would be simpler and I think would actually be pretty reasonable