test_speed_table: improve static assertion

Test against the resulting timer speed, not against the CPU clock

tests/unit/modules/speed_table/test_speed_table.cpp

@@ -5,8 +5,9 @@
 using Catch::Matchers::Equals;
 using namespace modules::speed_table;
 
-// The following reference values are calculated for 16MHz F_CPU
-static_assert(F_CPU == 16000000);
+// The following reference values are calculated for 2MHz timer
+static_assert(F_CPU / cpuFrequencyDivider == 2000000,
+    "speed tables not compatible for the requested frequency");
 
 static const st_timer_t reference[][3] = {
     { 1, 62500, 1 },