Unify code coverage

[anrossi]

Currently, the code coverage data is incomplete. The unit tests statically link the msquic library, to access the internal functions, which means that code coverage doesn't realize those functions are covered, because it measures coverage per-binary, and the non-unit tests dynamically link msquic.dll.

CMake has a build option which will export all functions in a library, WINDOWS_EXPORT_ALL_SYMBOLS. This can be used when building the code coverage version of the library so that the dll can be dynamically linked by the unit tests.