cleanup: Map test files according to structure in src

For example cmd_account() is located in src/command/cmd_funcs.c.
The unit test was located in tests/unittests/test_cmd_account.c.
Let's move it to a subdirectory like tests/unittests/command/test_cmd_account.c
to correpsond to the same structure.

tests/unittests/command/test_cmd_roster.h

@@ -0,0 +1,17 @@
+void cmd_roster_shows_message_when_disconnecting(void** state);
+void cmd_roster_shows_message_when_connecting(void** state);
+void cmd_roster_shows_message_when_disconnected(void** state);
+void cmd_roster_shows_message_when_undefined(void** state);
+void cmd_roster_shows_roster_when_no_args(void** state);
+void cmd_roster_add_shows_message_when_no_jid(void** state);
+void cmd_roster_add_sends_roster_add_request(void** state);
+void cmd_roster_remove_shows_message_when_no_jid(void** state);
+void cmd_roster_remove_sends_roster_remove_request(void** state);
+void cmd_roster_remove_nickname_sends_roster_remove_request(void** state);
+void cmd_roster_nick_shows_message_when_no_jid(void** state);
+void cmd_roster_nick_shows_message_when_no_nick(void** state);
+void cmd_roster_nick_shows_message_when_no_contact_exists(void** state);
+void cmd_roster_nick_sends_name_change_request(void** state);
+void cmd_roster_clearnick_shows_message_when_no_jid(void** state);
+void cmd_roster_clearnick_shows_message_when_no_contact_exists(void** state);
+void cmd_roster_clearnick_sends_name_change_request_with_empty_nick(void** state);