Separate the test suite into its own package. The reason is that we can
avoid circular dependency on ‘hspec-megaparsec’ and thus avoid keeping
copies of its source files in our test suite, as we had to do before.
Another benefit is that we can export some auxiliary functions in
‘megaparsec-tests’ which can be used by other test suites, for example in
the ‘parser-combinators-tests’ package.
Version of ‘megaparsec-tests’ will be kept in sync with versions of
‘megaparsec’ from now on.
