Swift: Make file checking in tests more strict

With Swift 6.1 the extractor will start to extract files outside of the test
directory. These files and their elements we do not want to see in our tests.

Test forgotten in https://github.com/github/codeql/pull/19344

swift/ql/test/library-tests/elements/location/location.ql

@@ -1,4 +1,5 @@
 import swift
 
 from Location l
+where exists(l.getFile().getRelativePath()) or l instanceof UnknownLocation
 select l