differentiate suite names in a parallel_tests context

When using the formatter with parallel tests (https://github.com/grosser/parallel_tests), we ran into issues with Jenkins dropping one or more of the test result files' data (i.e., failing to include it in reports/tallies), apparently because all of the `testsuite` elements have the same name (specifically, `rspec`).

To fix that issue, this PR transparently leverages the parallel_tests environment variable `TEST_ENV_NUMBER`, if it is present, to make the suite names unique (cf. https://github.com/grosser/parallel_tests#add-to-configdatabaseyml), by simply appending it.