tests: test garak's loading performance

[leondz]

garak should be fast until we're doing generation, probing, detection.

this means class instantiation should also be quick, and so loading has to be lazy

add a unit test that loads plugins and flags those loading "slowly"

slowly - 600ms?

performance is predicated on the test machine

if there are chunky imports that slow things down, then the test should fail, we want to know about this, we want to load lazier & let users move fast - garak should not be annoying to use

[leondz]

add speed test for the --list_generators / probes / detectors commands also