[Tests] Make OS features consistent

lit.py currently allows any substring of `target_triple` to be used as a
feature in REQUIRES/UNSUPPORTED/XFAIL. This results in various forms of
the OS spread across the tests and is also somewhat confusing since they
aren't actually listed in the available features.

Modify all OS-related features to use the `OS=` version that Swift adds
instead. We can later remove `config.target_triple` so that these don't
the non-OS versions don't work in the first place.

test/Concurrency/Runtime/async_taskgroup_is_asyncsequence.swift

@@ -7,7 +7,7 @@
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
 
-// UNSUPPORTED: linux
+// UNSUPPORTED: OS=linux-gnu
 
 @available(SwiftStdlib 5.1, *)
 func test_taskGroup_is_asyncSequence() async {