coverage: dist-hook and distcheck-hook can modify file permission

coverage: dist-hook and distcheck-hook can modify file permission

See automake bug#10878.

* tests/dist-hook-perms.test: New test, checking that the user can
use the 'dist-hook' target to modify permissions of distributed
files before putting them in the distribution tarball.
* tests/distcheck-writable-srcdir.test: New test, checking that the
user can use 'distcheck-hook' to ensure his packages is built by
"make distcheck" with a writable srcdir.  This goes against the GNU
Coding Standards, but some package authors might not care about
them too much, and it's nice to support their use case too.
* tests/list-of-tests.mk: Add them.