Switch make-check to run the short test suite by default

The set of C suite tests run by make check is controlled by an environment variable TESTUTIL_DISABLE_LONG_TESTS. That has caused problems in our automated testing, since many don't disable long tests.

The goal of make check is to run a quick test suite to validate an application. We should switch the logic to be TESTUTIL_ENABLE_LONG_TESTS. The current test only checks for the existence of the environment variable - we should probably also check to see if it's enabled or disabled (1 or 0).

We should review which tests are considered long, I believe this was first added to make it practical to run make check under valgrind. It's possible that we want more than an on/off switch for long tests.