docs: Document test suites for all three stacks

tests.yml now uses TESTIMAGE_AUTO to automatically run the tests as part
of the build step, so refactor the "Reproduce -> Validate" instructions
to instruct the user to run the build command and verify the log output.

Additionally, there are now test suites for the baremetal Zephyr and
virtualization stacks, so add correspond sections.

The Zephyr Kas file now builds all three sample applications by default,
so update the "Build and Run" instructions for baremetal Zephyr
accordingly.

Rename "Test Suite" in the Developer Manual to "Linux Test Suite" to
avoid confusion with the Yocto test suite.

Issue-Id: SCM-4957
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Ic5b8bb29b6b77f6576b38e7ca81e5008545e708e