Switch Test Framework to Catch2

[webern]

Better IDE support (CLion knows all about Catch2).

[webern]

Unfortunately this is bigger than I thought because I have inherited from the cpp unit light Test class here

mx/Sourcecode/private/mxtest/file/MxFileTest.h

Line 12 in 65545b4

class MxFileTest : public ::Test

[webern]

...though it may be possible to hack into catch2 in the same way:

mx/Sourcecode/private/cpul/catch.h

Line 4750 in b36e91c

class TestCase : public TestCaseInfo {