oeqa/selftest/rust: Various fixes to work correctly

oeqa/selftest/rust: Various fixes to work correctly

* Ensure the test/class naming doesn't allow the to be triggered
  without the toolchain decorator

* Add the toolchain-user decorator so it runs on non-IA targets

* Strip the leading "[XX] " prefix from the test names

* Ensure skipped test counts are passed through correctly (as SKIPPED, not SKIP)

* Avoid duplicate test results and show a warning if any are found (duplicates
  were from other sources in the end but the code remains sensible to have)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>