t/lib/common.pl should use absolute paths in @INC

t/lib/common.pl needs to run test files within a subdirectory, and hence
needs change directory into it. Changing a directory poses a problem when
running the tests, because @INC is typically '../lib'. The fix for this had
been to prepend '../../lib' to @INC, which is the correct relative path
while in the subdirectory.

However, the END block changes directory upwards (so that it can delete the
temporary directory), and in the process of deletion may end up lazy-loading
more modules. This will end up searching in ../lib first (ie outside the
build directory), which is wrong, and will cause problems if a directory lib/
exists up one level with files of the expected names (eg a lib/ directory
from a different version of Perl).

Problem identified and diagnosed by Daniel Dragan in RT #116971.