oeqa/ltp: rewrote LTP testcase and parser

The LTP test reporting appears to be a little fragile so I tried to make
it more reliable.

Primarily this is done by not passing -p to runltp, which results in
machine-readable logfiles instead of human-readable.  These are easier
to parse and have more context in, so we can also report correctly
skipped tests.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>