tests: add helper to determine when to skip expensive tests

tests: add helper to determine when to skip expensive tests

The logic set up in previous patch for exposing VIR_TEST_EXPENSIVE
to individual tests is as follows:

make check VIR_TEST_EXPENSIVE=0   => getenv("VIR_TEST_EXPENSIVE") sees "0"
make check VIR_TEST_EXPENSIVE=1   => getenv("VIR_TEST_EXPENSIVE") sees "1"
make check                        => getenv("VIR_TEST_EXPENSIVE") sees
either "0" or "1", based on configure options
cd tests; ./FOOtest               => getenv("VIR_TEST_EXPENSIVE") sees
whatever is in your environment (usually NULL, but possibly garbage)

Merely checking if VIR_TEST_EXPENSIVE is set in the environment
does the wrong thing; likewise, it is unsafe to assume the
variable will always contain a valid number.

As such, it helps to have helper functions, instead of making each
expensive test repeat the probe of the environment.

* tests/testutils.h (virTestGetExpensive): New prototype.
* tests/testutils.c (virTestGetExpensive): Implement it.
* tests/test-lib.sh (very_expensive_): Rename...
(test_expensive): ...and tweak to use VIR_TEST_EXPENSIVE.

Signed-off-by: Eric Blake <eblake@redhat.com>