Add a `fixedSource` parameter to `assertMacroExpansion`

[ahoppen]

The assertParse function has a fixedSource parameter to check that the application of Fix-Its produces valid results and it has been a valuable tool so far. assertMacroExpansion could gain the same parameter to check that the macro’s Fix-Its work as expected.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://113567656