Make MBED_ASSERT failures fail the unit test case. Without this, we
might not notice the assertion failure as it wouldn't be bubbled up to
googletest.

We set the --coverage flag globally in UNITTESTS/CMakeLists.txt. We
shouldn't need to also set it on mbed-stubs-platform, so remove it.

We have a central collection of "stub headers", which makes reasoning
about dependencies rather difficult, as it forces every stub library to
depend on all available stub headers. The standard approach would be for
each stub library to expose its public headers, and its dependents to
explicitly specify a dependency on the stub library containing the
headers it needs. This is a more modular design than creating a
header-only monolith library. Move the platform stub headers from this
central library into the mbed-stubs-platform library to increase
modularity.

mbed-stubs-connectivity now depends on the mbed-stubs-platform because
it requires some headers which were moved to mbed-stubs-platform.

Previously the platform stub library depended on `mbed-headers`, which
is a collection of all available headers in mbed-os. To make it easier
to separate the library, only depend on the headers we're actually using.

Move the header-only mbed-headers-platform library the unit test stubs
depend on into the platform component directory. This makes the platform
stubs more self contained and improves composition of the library.

Move the platform stub library into the platform component directory.
This change is so we can avoid duplicating the mbed-os source tree in a
central UNITTESTS folder.