Run timing selftest on all platforms

Used to fail on our FreeBSD and Windows buildbots. Seems to be working at
least on my Darwin physical machine and on my Windows VM with MSYS2 now.
diff --git a/programs/test/selftest.c b/programs/test/selftest.c
index 1aca4c1..405d0d8 100644
--- a/programs/test/selftest.c
+++ b/programs/test/selftest.c
@@ -262,8 +262,7 @@
 
 /* Slow tests last */
 
-/* Not stable enough on Windows and FreeBSD yet */
-#if __linux__ && defined(MBEDTLS_TIMING_C)
+#if defined(MBEDTLS_TIMING_C)
     if( ( ret = mbedtls_timing_self_test( v ) ) != 0 )
         return( ret );
 #endif