oeqa/runtime/parselogs: load ignores from disk

oeqa/runtime/parselogs: load ignores from disk

Instead of hardcoding the list of ignored errors/warnings in the test
itself, read them plain text files on disk.

This uses importlib to try to open a file called
oeqa.runtime.cases.parselogs-ignores-[candidate].txt, where the
candidate will be:

- "common"
- The TARGET_ARCH
- Each of the MACHINEOVERRDES

This allows the common and tune-specific ignores to be retained in
oe-core, and machine-specific ignores added to the layer where the
machine is defined.

[ YOCTO #14604 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>