Add integration tests for dyld interposition: DYLD_LIBRARY_PATH and
DYLD_INSERT_LIBRARIES.
DYLD_INSERT_LIBRARIES is also relevant for TSan thread
finalization/destruction sequence in the presence of additional pthread
introspection hooks (libBacktraceRecording.dylib for Xcode 'Queue
Debugging' feature).
rdar://78739125
Differential Revision: https://reviews.llvm.org/D109332
Julian Lettner