This adds support for templated tests and test methods via
`TEMPLATE_TEST_CASE` and `TEMPLATE_TEST_CASE_METHOD` macros. These
work mostly just like their regular counterparts*, but take an
unlimited** number of types as their last arguments.
* Unlike the plain `TEST_CASE*` macros, the `TEMPLATE*` variants
require a tag string.
** In practice there is limit of about 300 types.
Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)
The last three are enabled for Clang compilation going forward.
Also enabled `-Wunreachable-code` for Clang and GCC
