[validation-test] Add line-directive doctests

`utils/line-directive` is used when building Swift (see
`cmake/modules/AddSwift.cmake`), and includes some doctests. In order
to make sure it continues to behave as intended, add its tests to
`validation-test`.

Author: modocache

Diff for: validation-test/Python/line-directive.swift

@@ -0,0 +1 @@
+// RUN: %{python} %S/../../utils/line-directive