test/py: Always use the current dir as the source tree

test/py: Always use the current dir as the source tree

The logic in get_details() retrieves the default source directory from
the Labgrid settings. This is convenient for interactive use, since it
allows pytests to be run from any directory and still find the source
tree.

However, it is not actually correct.

Gitlab sets the current directory to the source tree and expects that to
be used. At present it is ignored. The result is that Gitlab builds
whatever happens to be in the default source directory, ignoring the
tree it is supposed to be building.

Fix this by using the directory of the source tree, always. This is
obtained by looking at the grandparent of the conftest.py file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Tom Rini <trini@konsulko.com>
Fixes: bf89a8f1fc2 ("test: Introduce the concept of a role")
Tested-by: Tom Rini <trini@konsulko.com>