When running unit tests on the ESP32, the current directory is root, while
on the PC (or in the GitHub Actions), the tests/ directory is the current
directory. Unit tests that rely on fixture files fails because of this.
Now test scripts can look for fixture files relative to their own path to
make them independent of the current directory.

This allows for comparing binaries generated on the ESP32 with those generated
on the PC with binutils-esp32ulp.

During testing on the ESP32, given that there is no binutils-esp32ulp, only the
py-esp32-ulp part of the compat tests can run. Calculating the SHA1 for each
resulting file from the compat tests, then allows comparing those hashes with
hashes calculated on the PC.

If code outside our main entry point, e.g. our examples, wanted
to use the src_to_binary function, it needed an ugly import of
esp32_ulp.__main__. With this change the import becomes cleaner,
e.g. `from esp32_ulp import src_to_binary`.

The original "main" method, which took a filename as input and
resulted in a ULP binary file as output, is now also more cleanly
available as assemble_file() inside the esp32_ulp module.

Fixes micropython#39.

This example makes the ULP turn on and off GPIO2 at regular intervals.
If one attaches an LED to GPIO2 and GND, it would blink with a period
of approximately 500ms for on and off each.

The example defines all needed RTC constants, so it is usable even
without a populated Defines DB.

The example ends with a loop showing the contents of two memory fields,
which the ULP code updates. This allows seeing that the ULP is working.
(Can be useful for running the example, even when there is no LED
connected to GPIO2).

Fixes micropython#12.

Also, what versions and devices everything was tested on.

Everything was tested all the way back to v1.10 today, and the
current code works correctly. However, MicroPython v1.11 has
a bug related to loading the ULP binary, so while py-esp32-ulp
produces correct binaries on that version, they can not actually
be used on the same version. To avoid explaining all this detail,
MicroPython v1.12 was chosen as the minimum supported version.

(Actually MicroPython v1.12 massively reduced its own memory
consumption, which makes it much better for py-esp32-ulp anyway,
especially when assembling larger sources.)

Fixes micropython#48.

Add detail on what is not supported. Change from "alpha" to "beta"
for expressing the relative quality level of this software.