Run Druntime's testsuite on Buildkite

[wilzbach]

Split-off from #291

[wilzbach]

This is blocked on druntime's testsuite requiring gdb 8.
rt_trap_exceptions_drt_gdb in particular.

[dlang-bot]

Thanks for your pull request, @wilzbach!

[wilzbach]

This is finally passing.
CC @ZombineDev - okay with patching out the test from druntime which requires gdb8 for now?

[PetarKirov]

Sure, just open an issue to add GDB 8, so we don't forget to reenable the test when possible.

[wilzbach]

Sure, just open an issue to add GDB 8, so we don't forget to reenable the test when possible.

Done -> #339