Filter: Remove per-sample filter freq update from test

[rsaxvc]

Summary

Skip reconfiguring the harmonic notch fundamental frequency on every sample.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

I diffed the test_notchfilter output before and after the patch - no difference.

Description

Since the filter frequency is already configured to source_freq, and has not been changed since, we can skip reconfiguring it. This more closely matches how the filter is used, and causes no change to the generated test CSV files.

Before: ~81 seconds
After: ~33 seconds

[github-actions]

Global Size Summary (Total Flash delta in bytes)

Built from bf99003 rebased onto f48fabc

Board	Periph	tracker	blimp	bootloader	copter	heli	iofw	plane	rover	sub
DrotekP3Pro	_	_	_	*	_	_	_	_	_	_
Durandal	_	*	*	_	*	*	_	*	*	*
Hitec-Airspeed	*	_	_	_	_	_	_	_	_	_
KakuteF7	_	*	*	_	*	*	_	*	*	*
MambaH743v4	_	*	*	_	*	*	_	*	*	*
MatekF405-CAN	_	_	_	*	_	_	_	_	_	_
MatekF405	_	*	*	_	*	*	_	*	*	*
MatekH743-bdshot	_	*	*	_	*	*	_	*	*	*
Pixhawk1-1M	_	*	*	_	*	*	_	*	*	*
disco	_	*	*	_	*	*	_	*	*	*
f103-GPS	*	_	_	_	_	_	_	_	_	_