[CK] increase time limit for fmha_bwd tests to prevent
timeouts (#8241)
## Motivation
Observed a CI failure due to fmha_bwd test timeout which never happened
before. Going to increase the time limit for the test to prevent any
further CI failures.
## Technical Details
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## Test Plan
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## Test Result
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## Submission Checklist
- [ ] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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