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.
Thanks to the changes to compilation model, the tests for benchmarking
macros have been made part of the normal test run. This means that
the only purpose these separately compiled tests served was to waste
CI time.
Now that Catch2 is a proper library, we can always build the full
library (comparatively minor slowdown) and the user can avoid
including benchmarking headers to avoid the compilation slowdown.
When running tests in parallel, CTest runs the tests in decreasing
order of cost (time required), to get the largest speed up from
parallelism. However, the initial cost estimates for all tests are
0, and they are only updated after a test run. This works on a dev
machine, where the tests are ran over and over again, because
eventually the estimates become quite precise, but CI always does
a clean build with 0 estimates.
Because we have 2 slow tests, we want them to run first to avoid
losing parallelism. To do this, we provide them with a cost estimate
manually.
