Reduced exec time of frequency tests

[vfdev-5]

Description:

• Reduced exec time of frequency tests

Test of Frequency test_frequency_with_engine_* take a lot of time, around 61.41s
This PR reduces total_tokens = 1200 // workers -> total_tokens = 400 // workers
Hope it does make tests flaky again...

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)