Individual libraries' `target_h` stub headers have now all been moved
from `mbed-headers-base` to `mbed-headers-<library>`.

Note: Even though headers previously in `target_h` are technically
stubs/fakes too, they are used by not only unit tests but also regular
libraries when compiled for unit tests, because no target-specific HAL
implementation exists in this case. In order for regular library
sources to pick up `target_h` headers, those headers must

* have the same names as regular headers
* appear first in include paths

This is why those headers are part of `mbed-headers-<library>` and not
`mbed-stubs-<library>`. Before this refactoring, `mbed-headers-base`
was the first in unit tests' include paths.

Previously a test executable was recognised as a single test by CTest.
However, test executables usually contain multiple test cases, the
results of the test cases should be individually reported. With our
previous setup we could miss test case failures that didn't cause the
executable to return an error code.

This commit uses gtest_discover_test to discover all test cases in a
test executable. This enables CTest to match test passes and failures
from the googletest binary output.

Update cellular device unit tests CMake depends on a mock, which is
required for the test instead of depending on the group of mbed-stubs,
mbed-headers, mbed_stub_headers, libraries.
Limit the cellular context unit test CMake target_sources to keep only
required sources

- Previously, all netsocket headers made as a part of the
mbed-headers-connectivity library. To make it easier to separate all
the netsocket headers into the separate mbed-headers-netsocket library
and keep them, as part of netsocket stubs CMake. This makes the
netsocket stubs more self-contained and improves the composition of the library.
- Update cellular, netsocket unit tests to link only with a mbed-headers-netsocket
library that it depends on instead of link with mbed-headers
which is a group of headers libraries.

- Previously all cellular header made as a part of the
mbed-headers-connectivity library. To make it easier to separate all
the cellular headers into the separate mbed-headers-cellular library
and keep them as part of cellular stub CMake. This makes the cellular
stubs more self-contained and improves the composition of the library.
- Update all unit test that depends on cellular headers with mbed-headers-cellular library

Move the connectivity cellular into the connectivity/cellular component
directory. So we can avoid duplicating the mbed-os source tree in a
central UNITTESTS folder.

- Add CMake configuration file into framework AT, device, common
unittest