tests: do not fail dispvm+thunderbird test early if TB is not installed

Checking if Thunderbird is installed is done later. Do not fail on
preparing its configuration - make it work also if /etc/thunderbird/pref
isn't pre-existing.

If TB is not installed, the test will be recorded as skipped anyway (so
the file won't be used), but create the dir anyway. This will allow
working with distributions not pre-creating that directory (if any).

Author: marmarek

qubes/tests/integ/dispvm.py

@@ -962,7 +962,7 @@ def test_100_open_in_dispvm(self):
         # disable donation message as it messes with editor detection
         self.loop.run_until_complete(
             self.testvm1.run_for_stdio(
-                "cat > /etc/thunderbird/pref/test.js",
+                "mkdir -p /etc/thunderbird/pref && cat > /etc/thunderbird/pref/test.js",
                 input=b'pref("app.donation.eoy.version.viewed", 100);\n',
                 user="root",
             )