tests/run-tests.py: Raise OSError on invalid file open for injected FS.

This makes sure that `open()` raises `OSError(ENOENT)` if the test script
attempts to open a file that does not exist.

For example, the test `tests/ports/esp32/check_err_str.py` attempts to do
this and this test currently fails when run with `--via-mpy` because the
injected filesystem lets a file by any name be opened.

The fix here tries to be as minimal as possible, so includes a size
optimisation for `stat()` to use a fixed 10-tuple.  It also fixes the
`OSError` argument, which should be positive.

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

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