tests: added certificate chain unit tests

We add a new intermediate CA and a new server certificate signed by this
new CA. Unit tests verify that the chain is loaded successfully and that
the old behavior is kept by loading a file with an invalid chain.

https://bugzilla.gnome.org/show_bug.cgi?id=729739
tls: Fix a leak in GTlsDatabaseGnutls

https://bugzilla.gnome.org/show_bug.cgi?id=737106
tls: Fix various leaks in the TLS unit tests

https://bugzilla.gnome.org/show_bug.cgi?id=737106
gnutls: Ensure all private members of GTlsConnectionGnutls are freed

Just to be on the safe side, ensure every private member of
GTlsConnectionGnutls is freed when the object itself is finalised.

implicit_handshake must always be NULL when finalize() is reached, as it
holds a reference to the GTlsConnectionGnutls as its source object.
However, it’s better to clear the member anyway, just in case this
behaviour changes in future.

https://bugzilla.gnome.org/show_bug.cgi?id=736809
Added Kazakh translation
gnutls: loosen the semantics of "use-ssl3"

If SSL 3.0 is disabled, then make "use-ssl3" mean "use the lowest
available TLS version" instead, so that, eg, TLS 1.2 -> TLS 1.0
fallback is still possible.

https://bugzilla.gnome.org/show_bug.cgi?id=738633
gnutls: update default priority string and fallback rules

Change the default "priority" string from "NORMAL:%COMPAT" to
"NORMAL:%COMPAT:%LATEST_RECORD_VERSION". (Correctly-implemented
servers won't care either way, but apparently some servers have been
incorrectly updated for POODLE-safeness by rejecting all handshakes
with SSL 3.0 in the version field [which had long been recommended for
compatibility reasons], even if the handshake offered to negotiate TLS
versions too.)

Change the fallback/"ssl3" priority code so that it never includes
%LATEST_RECORD_VERSION, but does include %COMPAT even if the default
priority doesn't (eg, because it was overridden with
G_TLS_GNUTLS_PRIORITY).

https://bugzilla.gnome.org/show_bug.cgi?id=740087
tls/tests: dist files/chain.pem
2.43.1