Now a `TEST_CASE` macro should create a single TestCaseInfo and then
it should never be copied around. This, together with latter changes,
should significantly decrease the number of allocations made before
`main` is even entered.
The JUnit report is improved in that:
* The message shows the testing condition, not the result
* The actual message has similar output than the console one
By opting the JUnit and XML reporters into it, we no longer run
into problem where they underreport the results without `-s` flag.
Related to #1264, #1267, #1310
ReusableStringStream holds a std::ostringstream internally, but only exposes the ostream interface.
It caches a pool of ostringstreams in a vector which is currently global, but will be made thread-local.
Altogether this should enable both runtime and compile-time benefits. although more work is needed to realise the compile time opportunities.
