Use Catch for our test suite

`make check` is also added to compile and run the tests
Catch is fetched with cmake automatically into the build directory when needed

Refactor XmppParser::end_element to make it clearer

Both for me, and apparently for clang static analyzer, who reported a (imo)
false positive.

Style, add a few “this->”

Fix some trivial issues reported by cppcheck

Fix a build issue when compiling without cares

Fix the jidprep tests when built without libidn

Test a different way to do our CI

Introduce a script, which is called with different parameters, to have a
number of builds done with a different configuration, without repeating
things too much.

Remove a unused_parameter warning

Remove some more debug thing

Introduce a basic .gitlab-ci.yml file

Remove some debug thing that should not have been committed

Display a success message at the end of the test suite

Use “using” instead of typedef

Remove an unused include

Some little fixes in test.cpp

Only compile database.cpp if configured with litesql

Add the network module to the test_suite

Because we added a network unit test

Separate the DNS resolution logic from the TCP communication logic

fix #3137

Introduce the realname_from_jid option

When set to true, the realname and username are extracted (by default) from
the user’s JID

fix #3136

Provide the “realname_customization” option

ref #3136