test: fix unit tests failing on repeated runs (#39915)

`go test -count=2` failed in 12 packages because tests leaked state into
later runs.

- Reset SQLite sequences after each fixture load, like PostgreSQL
already does
- Prepare the database in tests that relied on earlier tests doing it
- Restore mutated globals with `test.MockVariableValue`

modules/system/appstate_test.go

@@ -33,6 +33,8 @@ func (*testItem2) Name() string {
 }
 
 func TestAppStateDB(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
 	as := &DBStore{}
 
 	item1 := new(testItem1)