Fixes astroid bug #614.

This reverts commit fc407efe.

It's getting annoying from time to time as it seems to be flaky.

The result is usually static, but we're hitting this function quite
a lot for big projects such as pandas. This change is a matter of compromising
between the amount of memory astroid needs for analysis (e.g. storing the
results in memory) vs the wasted CPU time in calling the same
function over and over.

Uninferable acts as None so that should be enough.

This was initially added in e642ba33 so astroid won't
skip undesired inference branches. The method though doesn't seem to influence anything
as the added test is now passing regardless if we restore the path or not. The correct
approach would have been to clone the context instead. But given that the context is usually
used for caching already inferenced objects, we don't necessarily need to clone it here, but rather
in the low level inference, where we'd know that the inference path will diverge.

Remove the absolute to relative path transform for coverage, which is no longer needed as tests are not installed with the package anymore