Improve test coverage - multiple GDBs and Platforms

[jonahgraham]

There are some tests and functionality that are GDB version dependent. Like we have with DSF-GDB we should be running tests against all supported GDB versions. At the moment the whole testsuite takes only a few minutes to run - so adding all the supported GDB versions should not be too cumbersome. I don't recommend we change how much yarn test does, but instead add a new longer test.

• Is there a standard way to enable long running tests with yarn/mocha?
• provide way to specify which gdb version to use (this is partially there already with --gdb-path, but you can't today do yarn test --gdb-path gdb.x.y because the --gdb-path is not passed to each inferior test suite.
• add windows testing support - this is dependent on cdt-infra changes, see WIP
• ?? Add cross compilation test - probably use qemu arm and cross compilation toolchain (can we use what is in Ubuntu packages?)