tests: Don't advertise VIR_TEST_EXPENSIVE to users

tests: Don't advertise VIR_TEST_EXPENSIVE to users

Right now, the logic that takes care of deciding whether expensive
tests should be run or not is not working correctly: more
specifically, it's not possible to use something like

  $ VIR_TEST_EXPENSIVE=1 ninja test

to override the default choice, because in meson.build we always
pass an explicit value that overrides whatever is present in the
environment.

We could implement logic to make this work properly, but that
would require some refactoring of our test infrastructure and is
arguably of little value given that running

  $ meson build -Dexpensive_tests=enabled

is very fast, so let's just stop telling users about the variable
instead and call it a day.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>