Fix test_cuda failure: use BotorchTestCase for TestBetaPrior

[hvarfner]

Summary:
TestBetaPrior extends unittest.TestCase instead of BotorchTestCase, which causes test_cuda.py to hit a ValueError. The test_cuda dispatcher only handles BotorchTestCase, TestSuite, self-reference, and _FailedTest — plain unittest.TestCase hits the else branch and raises.

Switch to BotorchTestCase for consistency with the rest of the test suite.

Differential Revision: D100337776

[meta-codesync]

@hvarfner has exported this pull request. If you are a Meta employee, you can view the originating Diff in D100337776.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (db3a00a) to head (00a0a09).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3274   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         221      221           
  Lines       21926    21926           
=======================================
  Hits        21922    21922           
  Misses          4        4           

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