tests: speed up integration tests coverage

Previously, .simplecov called `SimpleCov.result` to store the coverage
result, and ignored the return value. `SimpleCov.result`'s return can be
slow to calculate, which wastes a lot of time when it's ignored.

This commit extracts the code needed to store the SimpleCov result from
`SimpleCov.result`, and calls it directly, without doing the busywork to
compute the return value every time.

In my testing, this more than halves the time taken to run all the
integration tests.

Library/Homebrew/.simplecov

@@ -20,7 +20,13 @@ SimpleCov.start do
     at_exit do
       exit_code = $!.nil? ? 0 : $!.status
       $stdout.reopen("/dev/null")
-      SimpleCov.result # Just save result, but don't write formatted output.
+
+      # Just save result, but don't write formatted output.
+      coverage_result = Coverage.result
+      SimpleCov.add_not_loaded_files(coverage_result)
+      simplecov_result = SimpleCov::Result.new(coverage_result)
+      SimpleCov::ResultMerger.store_result(simplecov_result)
+
       exit! exit_code
     end
   else