We start at version 2, with version 1 being taken up by the output
from Catch2 v2.

They also got slapped with the `[approvals]` tag in the process,
because we have too many approval tests and want less of them,
and these particular tests don't bring much value.

Related to #2090

This worsens the message for negative numbers a bit, but
simplifies the code enough that this is still a win.

There is an increasing number of places where Catch2 wants to parse
strings into numbers, but being stuck in C++14 world, we do not
have good stdlib facilities to do this (`strtoul` and `stoul`
are both bad).

This is kinda messy, because there is no good way to signal to
the compiler that some code uses direct comparison of floating
point numbers intentionally, so instead we have to use diagnostic
pragmas.

We also have to over-suppress the test files, because Clang (and
possibly GCC) still issue warnings from template instantiation
even if the instantion site is under warning suppression, so the
template definition has to be under warning suppression as well.

Closes #2406