TAMAYA-288: Additional test coverage

[peculater]

Looks huge and scary, but it's honestly just adding tests.

Things that this PR does:

• Significant increase in test coverage, especially mutation coverage
  ** API from 57% to 92%
  ** Core from 38% to 82%
  ** SPI-Support from ??% (mutation coverage disabled) to 86%
• Tests now succeed with OpenJDK, Hotspot, and OpenJ9.
• Generated a series of additional bug tickets. :-)

What this does not do:

• Touch any implementation code. This is only tests.
• Reduce code duplication, especially between API/Core and SPI-Support.
• Have a great answer for OSGI testing. It ends up being a pile of mocks.