Reset the progress to 0 when setting a non-action state

Per hughsie, when setting a non-action state, the apps' progress should
be reset to 0.

This patch adds that and also its coverage in the dummy plugin's tests.
1 job from wip/improve-tests in 3 minutes and 51 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
failed #196409
build-gnome-software

00:03:51

 
Name Stage Failure
failed
build-gnome-software Build
Gs:ERROR:../lib/gs-self-test.c:727:gs_app_list_func: assertion failed (gs_app_list_get_progress (list) == 75): (0 == 75)
1/1 gnome-software / gs-self-test-lib FAIL 0.12 s (killed by signal 6 SIGABRT)

OK: 0
FAIL: 1
SKIP: 0
TIMEOUT: 0

ERROR: Job failed: exit code 1