This patch fixes races in several of the VPI tests that led to different output order when run on other simulators. It also adds a "vstd" category to the VPI regression test list to cover tests where Icarus extends or deviates from the standard.
Note that after applying this patch, devel will fail the spec_delays test. I believe the original test (and consequently, the Icarus VPI implementation) is incorrect. I will be submitting a compiler bug report for this. I have created an exception in vpi_regress.list so that v0.9 does not fail this test, as backporting a fix for this may break user's code.