Disable parallel for small test coverage runs

`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. 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.

Library/Homebrew/dev-cmd/tests.rb

@@ -3,6 +3,7 @@
 
 require "abstract_command"
 require "fileutils"
+require "hardware"
 require "system_command"
 
 module Homebrew
@@ -75,7 +76,7 @@ def run
             end
           end
 
-          parallel = false if args.profile
+          parallel = false if args.profile || (args.coverage? && files.length < Hardware::CPU.cores)
 
           parallel_rspec_log_name = "parallel_runtime_rspec"
           parallel_rspec_log_name = "#{parallel_rspec_log_name}.generic" if args.generic?