tests(iid): Explicitly set fixed_irc_server

These tests require that fixed_irc_server is set, and will fail if
another test sets them.

This may not have been a problem before, but since the catch2 update CI
has encountered issues running these tests randomly, as the order of
which tests are ran in is now random.

tests/iid.cpp

@@ -7,6 +7,7 @@
 
 TEST_CASE("Irc user parsing")
 {
+  Config::set("fixed_irc_server", "", false);
   const std::map<char, char> prefixes{{'!', 'a'}, {'@', 'o'}};
   IrcUser user1("!nick!~some@host.bla", prefixes);
   CHECK(user1.nick == "nick");
@@ -23,6 +24,7 @@ TEST_CASE("Irc user parsing")
 
 TEST_CASE("multi-prefix")
 {
+  Config::set("fixed_irc_server", "", false);
   const std::map<char, char> prefixes{{'!', 'a'}, {'@', 'o'}, {'~', 'f'}};
   IrcUser user("!@~nick", prefixes);
   CHECK(user.nick == "nick");
@@ -47,6 +49,7 @@ namespace Catch
 
 TEST_CASE("Iid creation")
 {
+    Config::set("fixed_irc_server", "", false);
     const std::set<char> chantypes {'&', '#', '+', '!'};
     Iid iid1("foo%irc.example.org", chantypes);
     CHECK(std::to_string(iid1) == "foo%irc.example.org");