dnsdist: Add a regression test rewriting the response code via Lua

[rgacogne]

Short description

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 12236868095

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 59 unchanged lines in 13 files lost coverage.
• Overall coverage increased (+0.03%) to 64.785%

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Files with Coverage Reduction	New Missed Lines	%
pdns/packethandler.cc	1	72.57%
pdns/pollmplexer.cc	1	83.66%
ext/json11/json11.cpp	2	62.72%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
pdns/tsigverifier.cc	3	77.22%
pdns/iputils.cc	3	55.91%
modules/gpgsqlbackend/spgsql.cc	3	67.46%
pdns/recursordist/syncres.cc	4	79.36%
pdns/recursordist/lwres.cc	4	69.66%
pdns/recursordist/test-syncres_cc1.cc	6	89.34%

Totals
Change from base Build 12236241127:	0.03%
Covered Lines:	125938
Relevant Lines:	163527

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💛 - Coveralls