oeqa/selftest: OESelftestTestContext: convert relative to full path when newbuilddir...

oeqa/selftest: OESelftestTestContext: convert relative to full path when newbuilddir is provided

Relative paths in BBLAYERS only works when the new build dir are on the same
ascending directory node:
 .
 ├── build
 ├── build-st

It works because they share the same ascending relative directory node.

So use the full path when the argument newbuilddir is provided
to make the oe-selftest work everywere regardless of the location chosen.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>