Increased max time differences in tests to improve CI reliability.

Increased BOOST_THREAD_TEST_TIME_MS (a.k.a. max_diff) threshold that is
used in many tests for checking wakeup time delays to 600 ms on systems
other than Linux. This is to account increased delays on VMs that are
used to run CI on BSD systems, which occasionally cause excessively long
latencies in thread communication and wakeups.

Author: Lastique

test/timming.hpp

@@ -14,7 +14,7 @@
 #define BOOST_THREAD_TEST_TIME_MS 75
 #else
 // Windows, Cygwin, macOS all need this
-#define BOOST_THREAD_TEST_TIME_MS 400
+#define BOOST_THREAD_TEST_TIME_MS 600
 #endif
 #endif