improve test coverage

[totaam]

Split from #2356

[totaam]

Important: whenever we run a server or client as subprocess during the tests, the coverage tool does not record anything.. and maybe it should?

[totaam]

Despite hundreds of improvements and fixes to the unit tests, the coverage is only at 56%.

Here are the worst offenders:

Module	                                     statements	missing	excluded coverage
Total	                                         61696	27312	362	56%
xpra/scripts/main.py                             2353	1533	0	35%
xpra/server/window/window_video_source.py        1670	1185	0	29%
xpra/client/gtk_base/gtk_client_window_base.py   1555	969	0	38%
xpra/server/window/window_source.py              1711	828	0	52%
xpra/server/server_core.py                       1606	767	0	52%
xpra/net/ssh.py                                  746	678	0	9%
xpra/x11/server.py                               1042	652	0	37%
xpra/client/mixins/window_manager.py             1089	597	0	45%
xpra/client/gtk_base/gtk_client_base.py          1029	560	0	46%
xpra/net/file_transfer.py                        705	530	6	25%
xpra/x11/xkbhelper.py                            528	487	0	8%
xpra/platform/xposix/gui.py                      825	452	0	45%
xpra/x11/x11_server_core.py                      741	405	0	45%
xpra/client/gtk_base/gtk_tray_menu_base.py       1354	394	0	71%
xpra/x11/gtk_x11/clipboard.py                    532	363	0	32%
xpra/client/gl/gl_window_backing_base.py         867	353	0	59%

[totaam]

This will do for 4.1, will follow up in #2873.