Increase the default test duration a bit

There's a bunch of tests which are executed under the `virtual`
provision plan which can take a little bit more time. I suggest to
increase the default `duration` as I'm not sure it's really worth
to fine tune the value individually and `10m` sounds reasonable.
I believe we don't have to be that strict.

tests/main.fmf

@@ -4,6 +4,7 @@ contact: Petr Šplíchal <psplicha@redhat.com>
 tier: 2
 tag: [container, virtual]
 require: [tmt]
+duration: 10m
 
 adjust:
   - check: [avc]