swift234

[test] Add a failing test for SR-1129

SR-1129 tracks an issue in which the linking step fails if a base class,
defined in an imported module with a `public private(set) var`, is subclassed
by another module. This adds a test that XFAILs on Linux. The test
passed on OS X.

This issue is currently forcing swift-corelibs-xctest and
swift-corelibs-foundation to mark several privately-used variables as
`public`.