oeqa/parselogs: Don't use cwd for file transfers

oeqa/parselogs: Don't use cwd for file transfers

If you run:

MACHINE=A bitbake <image> -c testimage
MACHINE=B bitbake <image> -c testimage

and A has errors in parselogs, machine B can pick these up and cause
immense confusion. This is because the test transfers the log files
to cwd which is usually TOPDIR. This is clearly bad and this patch
uses a subdir of WORKDIR to ensure machines don't contaminate each
other.

Also ensure any previous logs are cleaned up from any existing
transfer directory.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>