frontend: fix broken tests

Some tests expected the WINDOWS_VM_NAME_MAX_LIMIT and
NON_WINDOWS_VM_NAME_MAX_LIMIT to be set, but they was set
from the AsyncDataProvider which is not inited in tests.

Added fallback to the default values.

Change-Id: Ia570c7e06f0c411025ae0a694225ccac83250d3f
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>