Test what symbols are exported

[mudge]

From @flavorjones:

verify what symbols are being exported by the precompiled C extension
I strongly suggest checking that one and only one symbol be exported, Init_re2
you can check this in a test, too: https://github.com/ruby/prism/blob/81265ed8a75806fc4f15f75f0f67b4795e535dde/test/prism/library_symbols_test.rb