Fixes for promotion to default gem

[kddnewton]

Two things are going wrong in the test suite for ruby core ruby/ruby#2631. This attempts to fix both.

1. assert_raises throws a NoMethodError from within ruby core. Not sure why? It says to use assert_raise so that what I've gone and replaced everything with. Test suite still passes just fine so I imagine it's an alias in some versions of test/unit and not in others. Weird.
2. Due to the nature of how ruby core is running the tests, the bundled ruby did_you_mean is higher in the path than this gem's lib directory. So when you require did_you_mean, you actually end up testing the bundled version instead of the source code. Because of that, this commit changes the requires to be relative so that we're guaranteed to be requiring the correct code. There doesn't really appear to be a standard way to do this within Ruby core itself, it looks like it's different depending on the gem.

For the second point there, I'm not sure if that means there's something that should be done in Ruby core, but this will certainly fix it for just DYM.

[yuki24]

This is awesome, thanks for fixing this! I just quickly had a look at https://bugs.ruby-lang.org/issues/10222 and require_relative looks good in 2.7, so it should be good to go.