mtest 0.8.0

• Where a case runs is stated by the case (only{}, except{}). A case or a suite names the
  platforms and architectures of mcpplibs.os it runs on (only{platform::windows},
  only{platform::linux, architecture::x86_64}) or not (except{platform::macos}); a suite's and
  its case's narrow together. Elsewhere it is skipped with one reason, only on windows (this host: linux x86_64). The Bash adapter (--only, --except), the Python adapter (only=,
  excepted=) and the C ABI (mtest_case_only, mtest_case_except) state the same; the engine
  skips a listed case outside its places without starting it.
• Code for one platform is selected when it is compiled. mtest re-exports mcpplibs.os's
  platform, architecture, can_run_on, host and on<P>: on<platform::windows>([&](auto) { ... }) is a body that a build for another platform discards, and an ok image decides when it is
  loaded.
• What a machine can do is declared in code and answered by the program (capability). The
  tests of [capabilities] -- a system, an architecture, programs on PATH, a file a glob matches, a
  command, environment values, other capabilities that must or must not hold -- and a function for
  what they cannot state, answered once per process. A capability is an attribute
  ("links"_test[gcc], with the tag requires:gcc); a case whose machine lacks it is skipped,
  missing capability: gcc (no g++ on PATH), under any runner (mtest, mcpp test, the program
  alone). The engine takes the program's answers: a word the program answers needs no
  [capabilities] entry, and the run records it.
• The listing says why a case is skipped, and where it runs (P3.3): a case event may carry
  reason (with expect: skip), only, except and capabilities; the schema states them. The
  conformance suite checks them of every level-3 implementation (placed).
• mtest ci coverage checks each case on its places: a case that states only must pass on
  each of its places that a collected run covered, and some run must have been on one -- what
  [ci.coverage] hosts states of a program, stated by the case.
• Additive: a program, a listing and a configuration that state none of these mean what they did.