dnsdist: Document that RE2 does full matching, add regression tests

[rgacogne]

Short description

Closes #15099

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 13114457556

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 760 unchanged lines in 24 files lost coverage.
• Overall coverage decreased (-0.01%) to 64.702%

---

Files with Coverage Reduction	New Missed Lines	%
pdns/misc.cc	1	62.6%
pdns/pollmplexer.cc	1	83.66%
pdns/recursordist/sortlist.cc	2	74.12%
pdns/tcpiohandler.cc	2	68.3%
modules/lmdbbackend/lmdbbackend.cc	2	73.65%
pdns/inflighter.cc	3	90.48%
pdns/iputils.cc	3	56.99%
modules/gpgsqlbackend/spgsql.cc	3	67.94%
pdns/pdnsexception.hh	3	25.0%
pdns/communicator.hh	5	77.78%

Totals
Change from base Build 13112383883:	-0.01%
Covered Lines:	127874
Relevant Lines:	166538

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💛 - Coveralls

[miodvallat]

^Looking good.$
^Don't forget to update the spell checker list of odd words as well.$