autotools: add lcov switch

This allows test coverage tests to be done in-tree, without the need for
the pull-tester.

./configure --enable-lcov
make
make cov
autotools: add scripts for pull-tester

Keep these scripts in-tree so that they can adapt without the need to try to
stay in sync.

These need a good audit to make sure they're still testing as much as before.
autotools: fix build race