Fixes #1800

b77cec05c0 fixed this problem for tagging tests, so that a test
case tagged with `[.foo]` would be parsed as tagged with `[.][foo]`.
This does the same for the test spec parsing.

Fixes #1798

Copying a `ReusableStringStream` would lead to "double free" of
the stream, and thus it could be used in multiple places at the
same time, breaking the output.

On systems where std::chrono::steady_clock::period is not std::nano, benchmark tests fail to compile due to trying to convert analysis.samples from a vector of duration<double, clock::period> to a vector of std::chrono::duration<double, std::nano>.

Rework how warning suppression in macros is done

Previously, each warning suppression was self-contained, with its
own pair of `SUPPRESS_X_WARNING` and `UNSUPPRESS_X_WARNING` macros.
This had the obvious advantage of being self-containing, but it
also meant that if we needed to suppress more than one warning
in a single place, then we would manipulate the compiler's warning
state multiple times, even though logically we would only need one
layer.

The new way of suppressing warnings in macros is to push compiler's
warning state with `CATCH_INTERNAL_START_WARNINGS_SUPPRESSION` macro,
then disable whatever macros we need with the
`CATCH_INTERNAL_SUPPRESS_X_WARNINGS` macro, and then return to the
previous state using `CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION`.