Explicit run mode and `--target` for CI tests

[ilammy]

This is a demonstration of issue #62.

I'm not really sure what do you want to do with these tests. Right now they are broken and it's not trivial to fix them. Might as well leave them broken as a reminder, or remove them altogether from ci.yml to be re-added when they actually work.

[jyn514]

These were added in https://github.com/cloudflare/boring/commit/ec45baa6d85cfb964b5c0db9a94aad2b5a48259c. @inikulin I think I probably lean towards removing them for now - do you have a strong opinion? We still have tests on the host platform, and it seems pretty difficult to try and get this working when cross-compiling ...