Refactor jet_tests.cc

1. Break tests for one function into its own TEST instance.
2. Use a gmock matcher which gives better logging, previously
   we were using a function which in turn used another function
   in test_utils. It made tracking down the failing test very
   hard.
3. Disable a hypot3 underflow and overflow test when using
   libc++ as the three argument in libc++ is borked.
4. The use of the matcher has made a number of comments redundant
   so I have removed them.

This is the first step in cleaning up these tests and then expanding
them.

Change-Id: Ib0c827f44432e2496e50b8cda3e06c26bdf50f08