Re-enable tests working on Linux

[augusto2112]

This PR re-enables tests TestSwiftConditionalBreakpoint and TestSwiftStepping, which are currently disabled on Linux, but pass successfully.

[dcci]

@adrian-prantl I don't see a reason why these two should be disabled, you? I think the only problem might be they're flakey, but we're going to figure this out soon, I believe.

[dcci]

@swift-ci test

[adrian-prantl]

What does git blame say? I suppose that stepping could be sensitive to codegen? Let's check all the various ubuntu variant bots after landing this. If they work, we'll leave them on.

[augusto2112]

I took a look at the history of both tests, they were set to skip Linux on 21/08/2018. Message is not very informative, just: "Merge lldb: Merge remote-tracking branch 'origin/stable' into upstream-with-swift"

[augusto2112]

@dcci and @adrian-prantl just reminding you guys about this :)

[adrian-prantl]

Can you also create the same PR for the swift/master-next branch? (just cherry-pick the changes and created a second PR, no need to re-test it)

[augusto2112]

@adrian-prantl already did it :) #972

[adrian-prantl]

Can you please take a look at all three Linux bots on ci.swift.org tomorrow and double-check that none of the newly enabled tests are failing? It could happen that it fails on only one Ubuntu variant, and swiftci test only tests one of them.

[augusto2112]

Ok, I'll double check tomorrow.