mgr/dashboard: Resolve TestBed performance issue

mgr/dashboard: Resolve TestBed performance issue

With this helper function you can easily resolve the TestBed resetting
performance issue. If more tests exists in a test suite, it makes sense
to configure TestBed only once if you are not doing a lot of TestBed
specific stuff (haven't hit the limitation). It will reduce the test
run time by around $tests * 50 %. In my case it was a test suite with
47 tests with a run time of over 30s after using the static test bed
method it ran in 1.2s. The run time was reduced to 0.04 %! This is
equivalent to a speed increase of 2500% (100/0.04)!

For our own security the normal way will be taken if you not
set the _DEV_ configuration variable to true. It will be false when
"run-frontend-unittests.sh" is run.

Signed-off-by: Stephan Müller <smueller@suse.com>