[test] Modify run-test to allow unified LLVM builds

[drodriguez]

The unified LLVM builds use a slightly different structure than the
Swift standalone builds. Modify the run-test utility tool to allow using
it in both standalone and unified builds.

The current Windows building instructions (https://github.com/apple/swift/blob/master/docs/WindowsBuild.md) recommend an unified build, and run-test will not work for people using those instructions. The unified builds are also possible in Linux and other platforms, but the instructions there are using the build-script, which will perform standalone builds.

[drodriguez]

@swift-ci please smoke test

[compnerd]

I dont have any objections to this, just curious. Are there users of run-test currently?

[drodriguez]

I do. I imagine that others that have modified the file over time also do. I don't think any infra uses it (and I don't think it is mentioned in any documentation, it is kind a secret). It is useful to me because you type less, you will never forget to recompile before running the tests, it is easier to filter tests out, uploads the artifacts to Android devices automatically, and some other niceties.

[compnerd]

I guess the other question is, why not invert the flag? Make it true by default

[rintaro]

I am also a heavy user of run-test. I have no objection to this as long as this doesn't change the default behaviors.

But Isn't it possible to auto-detect the unified build instead of manually specifying --unified option? I.e. if there's $build_dir/tools/test-$target can we say it is a unified build?

[drodriguez]

Re: "Why not invert the flag?". Because most of the people use build-script, and build-script do not build unified at all. This allows people that were using run-test to keep going without any modifications to their workflows. If unified someday becomes the standard, we can consider inverting the flag.

Re "auto-detect the unified build": I would prefer to be explicit, but I can change it to autodetect if people prefer so (I hope my layout is the same as other people's layouts).