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

• [x] Closes #7985
• [ ] Related to #

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] 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

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