G_APPLICATION_FLAGS_NONE was deprecated, but we don't actually need the single
instance functionality, so just use a non-deprecated flag instead.
This fixes Debian CI.
Note that g_assert() should not be used in unit tests, since it is a
no-op when compiling with G_DISABLE_ASSERT. Use g_assert() in production
code, and g_assert_true() in unit tests.
See https://github.com/fwupd/fwupd/issues/3790
When using the sync API entrypoints like fwupd_client_get_devices() these call
into fwupd_client_connect() to ensure the proxy is set up. This is not thread
safe and chaos ensues if you call two sync functions from different threads,
like GNOME Software does at startup...
Use a mutex to protect access to this shared resource.
Original test code from Philip Withnall, slightly tweaked by me. Thanks!
