Improve error robustness of unit tests

[Emrys365]

What?

This PR improves two unit tests to avoid occasional errors caused by numerical precision or randomness.

• test/espnet2/asr/frontend/test_frontend.py: use a predefined 2-ch signal instead of a randomly sampled signal for beamforming to avoid singular covariance matrix.
• test/espnet2/enh/layers/test_complex_utils.py: fix the random seed to avoid occasional error when the numerical difference is slightly larger than 1e-06.

Why?

Sometimes we may observe errors as in https://github.com/espnet/espnet/actions/runs/6719461822/job/18261148245 which are essentially numerical issues. After the modifications in this PR, these errors should be avoided in most cases.

[codecov]

Codecov Report

Merging #5523 (eb6f93a) into master (389b9cd) will increase coverage by 0.00%.
Report is 11 commits behind head on master.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5523   +/-   ##
=======================================
  Coverage   75.43%   75.43%           
=======================================
  Files         711      711           
  Lines       65757    65757           
=======================================
+ Hits        49606    49607    +1     
+ Misses      16151    16150    -1     

Flag	Coverage Δ
test_configuration_espnet2	∅ <ø> (∅)
test_integration_espnet1	65.67% <ø> (ø)
test_integration_espnet2	48.51% <ø> (ø)
test_python_espnet1	19.03% <ø> (ø)
test_python_espnet2	51.61% <ø> (+<0.01%)	⬆️
test_utils	23.10% <ø> (ø)

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[sw005320]

Thanks, @Emrys365!