Avoid unexpected heap growth in gctest caused by GC_disable

Avoid unexpected heap growth in gctest caused by GC_disable
(fix of commit 8fc1ebf6d)

Issue #264 (bdwgc).

This commit eliminates the scenario when one thread disables garbage
collections while other threads intensively allocate memory objects.

* tests/test.c (run_one_test): Do not call GC_disable and GC_enable.
* tests/test.c (run_single_threaded_test): New function (which tests
GC_disable and GC_enable).
* tests/test.c (main): Call run_single_threaded_test() (before
check_heap_stats).