Disable layout_string_witnesses_dynamic.swift

[drexin]

rdar://151476435

The test is currently crashing on Amazon Linux 2, so until we have a solution, we are disabling it.

[finagolfin]

Would be better to disable this for AL2 alone then.

[AnthonyLatsis]

 ******************** TEST 'Swift(macosx-x86_64) :: Interpreter/layout_string_witnesses_dynamic.swift' FAILED ********************
09:19:48  couldn't parse text: '://151476435'
09:19:48  in expression: 'rdar://151476435'
09:19:48  in REQUIRES: directive on test line 20

[AnthonyLatsis]

#81636