This commits also adds a script that does the amalgamation of headers
and .cpp files into the distributable version, removes the old
`generateSingleHeader` script, and also adds a very simple compilation
test for the amalgamated distribution.
With CMake 3.18.0 the `add_test(NAME)` handling changed. The escaped
double quotes confuse the new call. Work around this upstream change.
fixes: https://github.com/catchorg/Catch2/issues/1984