Added eigenvalue-based tests for PSD in stationary kernels

[fonnesbeck]

Description

Adds Rayleigh quotient-based tests of the power spectral density methods in all stationary covariance functions.

Related Issue

• Closes Improve tests for power spectral density methods in GP covariance functions #7985
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.49%. Comparing base (ebd836e) to head (3b54a5b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7986   +/-   ##
=======================================
  Coverage   91.49%   91.49%           
=======================================
  Files         116      116           
  Lines       18972    18972           
=======================================
  Hits        17359    17359           
  Misses       1613     1613           

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