A couple unit tests are infinite-looping when running against scala 2.13

[erikerlandson]

I disabled these in #759, with comments.

[gabrieljones]

https://github.com/typelevel/spire/blob/6d2409be57948f3d37448de78e95c64e3a8db871/tests/src/test/scala/spire/math/ComplexCheck.scala#L74-L78 Found this one. Where are the others?

[gabrieljones]

https://github.com/typelevel/spire/blob/6d2409be57948f3d37448de78e95c64e3a8db871/tests/src/test/scala/spire/math/AlgebraicTest.scala#L273-L287

Found a second reference to spire.scalacompat.preScala2p13

[kschwarz1116]

Attempted to address since the issue linked from the PR is fixed, however I seem to be running into a similar issue with scala 3 instead. I wonder if that change never made it into dotty. I'll keep digging.