Reduce number of test points in slow logcdf methods

[ricardoV94]

CI tests for logcdf methods that make use of dist_math.incomplete_beta::incomplete_beta are crazy slow. This is reported in #4420

This PR simply reduces the number of random tested points, which could save up to an hour in the CIs. Note that a similar change was already done for the logcdf method of the StudentT in #3823 for the same reasons.

Besides, these methods have passed many times now without failures, so it should be okay to reduce the number of points in each run. Objections?

Depending on what your PR does, here are a few things you might want to address in the description:

• what are the (breaking) changes that this PR makes? None
• important background, or details about the implementation. Trivial
• are the changes—especially new features—covered by tests and docstrings? Not needed.
• linting/style checks have been run
• consider adding/updating relevant example notebooks NA
• right before it's ready to merge, mention the PR in the RELEASE-NOTES.md. Not needed

[michaelosthege]

Absolutely!