tests/run-tests: Use absolute paths where possible.

Replace some usages of paths relative to the current working directory
with absolute paths relative to the tests directory.

Fixes and resulting changes:
- default values of MICROPYTHON and MPYCROSS are absolute paths and
  always correct
- likewise, the correct full paths for tools and extmod directories
  are appended to sys.path
- printing/cleaning failures works properly since it expects the .exp
  and .out files in the tests directory which is also where they
  are written to now, plus no more need for changing directories

This fixes #5872 and allows running custom tests which use run-tests
without having to cd to the tests directory first, and the test output
still is in the tests/ directory instead of the current working directory.

Discovery of tests and all skip test logic based on paths relative to
the current working directory remains unchanged which essentially means
that for running most of MicroPython's own tests, run-tests must still
be ran from within it's directory, so document that.

0 files changed, 0 insertions, 0 deletions