Add tests around phonetic similarity

[vickumar1981]

The tests for the phonetic similarities - soundex and metaphone are sparse. Need to add more tests/examples to make sure those are working as expected.

https://github.com/vickumar1981/stringdistance/blob/master/src/test/scala/fixtures/TestSoundCases.scala#L8

Can also enable/use (currently, disabled for phonetic similarities only) the coverage report to see what part of the algorithms is untested.

[vickumar1981]

Addressed by #53