Create the hs_test_helpers.{c|h} files that contains helper functions to
create introduction point, descriptor and compare descriptor.

Used by both the hs cache and hs descriptor tests. Unify them to avoid code
duplication.

Also, this commit fixes the usage of the signing key that was wrongly used
when creating a cross signed certificate.

Signed-off-by: David Goulet <dgoulet@torproject.org>