test-network: Remove/replace non-capturing group regex

test-network: Remove/replace non-capturing group regex

The systemd-networkd-tests.py has some regex that uses non-capturing
groups, but there is no need to use that with assertRegex; the
groups aren't referenced so it doesn't matter if it's capturing or
non-capturing.  However, there are a few places where optional groups
should have been used instead, so this changes that.

Specifically, groups like this:
(?:whatever |)
should actually be:
(whatever )?

Additionally, this is specifically needed for these tests to run on
Debian systems, because this assertRegex:
'Link File: (?:/usr)/lib/systemd/network/99-default.link'
needs to be:
'Link File: (/usr)?/lib/systemd/network/99-default.link'