Migrate from GoogleTest to Catch2

[chfast]

Replace the GoogleTest (gtest) unit test library with Catch2 or some other alternative.

Tests in Catch2 looks simpler than in GoogleTest.

• Parametrized tests. E.g. imagine writing the same state transition test for a set of EVM revisions. This is impractical in GoogleTest and the only option is to use a loop.
  • gtest: http://google.github.io/googletest/reference/testing.html#TEST_P
  • catch2: https://github.com/catchorg/Catch2/blob/devel/docs/generators.md
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• Tags
• BDD. Seems very useful for complex state tests: e.g. call a contract, then it selfdestructs, then it is called again...