tests: treat all truthy env vars the same way

tests: treat all truthy env vars the same way

Introduce a new class, that returns the truthiness of a env var.
Since an environment variable is just a string, it would normally
be true if not unset. The new class returns true when the env var is
set to a string that would be considered true.

Type: test
Depends-on: https://gerrit.fd.io/r/20484

Change-Id: I90ef010156f6fec246bde5c0e208ced1869b180f
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>