tests/run-tests.py: Change --target/--device options to --test-instance.

Previously to this commit, running the test suite on a bare-metal board
required specifying the target (really platform) and device, eg:

    $ ./run-tests.py --target pyboard --device /dev/ttyACM1

That's quite a lot to type, and you also need to know what the target
platform is, when a lot of the time you either don't care or it doesn't
matter.

This commit makes it easier to run the tests by replacing both of these
options with a single `--test-instance` (`-t` for short) option.  That
option specifies the executable/port/device to test.  Then the target
platform is automatically detected.

The `--test-instance` can be passed:
- "unix" (the default) to use the unix version of MicroPython
- "webassembly" to test the webassembly port
- anything else is considered a port/device to pass to Pyboard

There are also some shortcuts to specify a port/device, following
`mpremote`:
- a<n> is short for /dev/ttyACM<n>
- u<n> is short for /dev/ttyUSB<n>
- c<n> is short for COM<n>

For example:

    $ ./run-tests.py -t a1

Note that the default test instance is "unix" and so this commit does not
change the standard way to run tests on the unix port, by just doing
`./run-tests.py`.

As part of this change, the platform (and it's native architecture if it
supports importing native .mpy files) is show at the start of the test run.

Signed-off-by: Damien George <damien@micropython.org>

docs/develop/gettingstarted.rst

@@ -278,7 +278,7 @@ To run a selection of tests on a board/device connected over USB use:
 .. code-block:: bash
 
    $ cd tests
-   $ ./run-tests.py --target minimal --device /dev/ttyACM0
+   $ ./run-tests.py -t /dev/ttyACM0
 
 See also :ref:`writingtests`.
 

docs/develop/writingtests.rst

@@ -60,7 +60,7 @@ Then to run on a board:
 
 .. code-block:: bash
 
-   $ ./run-tests.py --target minimal --device /dev/ttyACM0
+   $ ./run-tests.py -t /dev/ttyACM0
 
 And to run only a certain set of tests (eg a directory):