filter: Accept rounded output from fir_filter_fsf

[argilo]

Description

VOLK recently accepted gnuradio/volk#709, which fixes truncate-toward-zero distortion in volk_32f_x2_dot_prod_16i. Unfortunately, this causes test failures:

======================================================================
FAIL: test_fir_filter_fsf_001 (__main__.test_filter)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/argilo/prefix_311/src/gnuradio/gr-filter/python/filter/qa_fir_filter.py", line 173, in test_fir_filter_fsf_001
    self.assertComplexTuplesAlmostEqual(expected_data, result_data, 5)
  File "/home/argilo/prefix_311/src/gnuradio/gnuradio-runtime/python/gnuradio/gr_unittest.py", line 73, in assertComplexTuplesAlmostEqual
    return all([
  File "/home/argilo/prefix_311/src/gnuradio/gnuradio-runtime/python/gnuradio/gr_unittest.py", line 74, in <listcomp>
    self.assertComplexAlmostEqual(x, y, places, msg)
  File "/home/argilo/prefix_311/src/gnuradio/gnuradio-runtime/python/gnuradio/gr_unittest.py", line 37, in assertComplexAlmostEqual
    raise self.failureException(
AssertionError: 1 != 2 within 5 places

======================================================================
FAIL: test_fir_filter_fsf_002 (__main__.test_filter)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/argilo/prefix_311/src/gnuradio/gr-filter/python/filter/qa_fir_filter.py", line 188, in test_fir_filter_fsf_002
    self.assertComplexTuplesAlmostEqual(expected_data, result_data, 5)
  File "/home/argilo/prefix_311/src/gnuradio/gnuradio-runtime/python/gnuradio/gr_unittest.py", line 73, in assertComplexTuplesAlmostEqual
    return all([
  File "/home/argilo/prefix_311/src/gnuradio/gnuradio-runtime/python/gnuradio/gr_unittest.py", line 74, in <listcomp>
    self.assertComplexAlmostEqual(x, y, places, msg)
  File "/home/argilo/prefix_311/src/gnuradio/gnuradio-runtime/python/gnuradio/gr_unittest.py", line 37, in assertComplexAlmostEqual
    raise self.failureException(
AssertionError: 5 != 6 within 5 places

This is because the tests in question assert the truncate-toward-zero behaviour in fir_filter_fsf. I would consider this behaviour buggy, and I would be surprised if there's any code that depends on fir_filter_fsf working this way. So I propose to change the tests to assert rounded output, with a temporary fallback to truncated output which can be removed once GNU Radio requires VOLK >= 3.1.0.

Which blocks/areas does this affect?

• Decimating FIR Filter (Float->Short variant only)
• Interpolating FIR Filter (Float->Short variant only)

Testing Done

I verified that the updated tests pass before & after the VOLK change.

Checklist

• I have read the CONTRIBUTING document.
• I have squashed my commits to have one significant change per commit.
• I have signed my commits before making this PR
• My code follows the code style of this project. See GREP1.md.
• I have updated the documentation where necessary.
• I have added tests to cover my changes, and all previous tests pass.

[argilo]

I also changed the assertions from assertComplexTuplesAlmostEqual to assertEqual, since the outputs are integers, not complex numbers.

[argilo]

The test failures are unrelated; they're both due to #6981.