tests: fix test coverage reporting for parallel testing.

* Fix simplecov clobbering test coverage reports when using parallel tests.
* Generate coverage reports by default (remove $SIMPLECOV flag).
* Store coverage reports in tmp/coverage/ instead of coverage/.
* Enable branch coverage.

ref: github.com /colszowka/simplecov/issues/718#issuecomment-538201587

.simplecov

@@ -0,0 +1,6 @@
+SimpleCov.start "rails" do
+  add_group "Libraries", ["app/logical", "lib"]
+  add_group "Presenters", "app/presenters"
+  enable_coverage :branch
+  coverage_dir "tmp/coverage"
+end