[Test] Add @escaping to async refactoring tests

The async refactorings ignore whether a completion handler had
`@escaping` or not. In preparation of fixing this, fix up all functions
to have `@escaping` for their completion handler parameter.

Also some small miscellaneous fixes in order to reduce the number of
warnings output on test failures and also the addition of `REQUIRES:
concurrency` on all tests.

test/SourceKit/Refactoring/basic.swift

@@ -116,7 +116,7 @@ HasInitWithDefaultArgs(y: 45, z: 89)
 func `hasBackticks`(`x`: Int) {}
 `hasBackticks`(`x`:2)
 
-func hasAsyncAlternative(completion: (String?, Error?) -> Void) { }
+func hasAsyncAlternative(completion: @escaping (String?, Error?) -> Void) { }
 func hasCallToAsyncAlternative() {
   hasAsyncAlternative { str, err in print(str!) }
 }