On vx4, test_thdn_lowpower fails on xcore when running on Jenkins

[shuchitak]

I've not been able to recreate these failures locally.

These tests fail on Jenkins:
test_thdn.py::Test_BasicMicArray::test_thdn_lowpower[300hz-1stg-python_xcore]
test_thdn.py::Test_BasicMicArray::test_thdn_lowpower[300hz-2stg-python_xcore]
test_thdn.py::Test_BasicMicArray::test_thdn_lowpower[5000hz-1stg-python_xcore]
test_thdn.py::Test_BasicMicArray::test_thdn_lowpower[5000hz-2stg-python_xcore]

As a workaround, I noticed that reducing test duration to 2seconds from 7seconds makes the tests pass, while still ensuring the THDN is good (very small durations are unrealistic for THDN calculation to get a good THDN).
This change is made as part of #287