[ng] coverage: cleaning rules with lots of files to clean

[ng] coverage: cleaning rules with lots of files to clean

* t/clean-many.sh: New test, try cleaning ~ 130 thousands files.
Currently fails on some systems (e.g., NetBSD 5.1).
* t/clean-many2.sh: New test, check that our cleaning rules do not
hit errors due to an exceeded command line length when there are
many files to clean; it does so by faking  a very low command line
length limit for 'rm' (max 50 arguments).  Currently failing.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>