Convert all tests to Googletest framework

[robertsj]

Googletest is incorporated in PR #51. With it in place, all existing tests should be switched to that format. This further simplifies the code base.

There should be a separate PR for each of these items:

• [x] utilities
• [x] angle
• [x] material
• [x] callow
• [x] geometry
• [x] boundary
• [x] external_source
• [x] kinetics
• [x] transport
• [ ] solvers
• [ ] ioutils
• [ ] postprocess
• [ ] orthog

At the top of each test directory CMakeLists.txt should be a block that looks like

macro(test)
  add_executable(${ARGV0} ${ARGV0}.cc)
  target_link_libraries(${ARGV0}
    geometry utilities angle
    GTest::gtest_main
  )
  gtest_discover_tests(${ARGV0})
endmacro()

Here, geometry utilities angle should be changed to the correct libraries for the tests.