Attempt to get tests passing on FreeBSD.

I'm currently working on sorting out core.time's tests so that they
should work on all platforms, but until then, I'm disabling this test on
FreeBSD so that the auto tester can pass.

src/core/time.d

@@ -1626,7 +1626,7 @@ struct TickDuration
     //can be devised and tested on a Windows box.
     //The differences in ticksPerSec on Windows makes testing
     //exact values a bit precarious.
-    version(Posix) unittest
+    version(linux) unittest
     {
         foreach(units; _TypeTuple!("seconds", "msecs", "usecs"))
         {