Small test fix for an update to pyuvdata's handling for rdate in uvfits

Description

This is a small fix for a test that started erroring as a result of a fix in pyuvdata's handling of the UVFITS parameter rdate.

This is inconsequential as far as pyuvsim functionality goes.

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Reference simulation update or replacement
[ ] Documentation change (documentation changes only)
[ ] Version change
[ ] Build or continuous integration change

For all pull requests:

Bug fix checklist:

[ ] My fix includes a new test that breaks as a result of the bug (if possible).
[x] All new and existing tests pass.
[ ] I have checked that I reproduce the reference simulations or if there are differences they are explained below (if appropriate). If there are changes that are correct, I will update the reference simulation files after this PR is merged.
[ ] I have checked (e.g., using the benchmarking tools) that this change does not significantly increase typical runtimes. If it does, I have included a justification in the comments on this PR.
[ ] I have updated the CHANGELOG.