tests/valgrind: isolate unreliable tests
tests/valgrind: isolate unreliable tests

jobstabwidget: I'm seeing around 20% failure with
  - 16 bytes indirect loss in QLineEdit::setText
  - 304 bytes direct loss in QWidgetLineControl::internalSetText
  These are likely to be related, as I haven't seen only one of them occur.

mainwindow: I'm seeing a 10% failure:
  - 320 bytes in QNetworkAccessManager (not cleaning up a thread it
    spawns internally).

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui