scripts: Support composite command in test maps

Some tests are side-effectful and must therefore be excecuted in
a particular order. Add a composite command element to group such
tests. Composite tests can contain port tests and and reboot
commands. The latter should no longer occur outside composite
tests.

Bug: 236898842
Change-Id: I28569a3ae4a819b3cdf2c39fea4c9125e33551ea
4 files changed