Try to make usleep test more robust (WebAssembly#681)

Match clock_nanosleep where `<=1` is tested, not `>=0`. See if this
fixes spurious failures.

Author: alexcrichton

test/src/usleep.c

@@ -32,7 +32,7 @@ int main(void)
                         / ns_to_sleep;
 
     // Allow the actual sleep time to be twice as much as the requested time
-    TEST(difference >= 0 && difference <= 1);
+    TEST(difference <= 1);
 
     return t_status;
 }