Allow bigger difference between timestamps in BasicsTests.Timestamps

[Gor027]

Increased the acceptable time difference between sequential inserts in BasicsTests.Timestamps test, cause it happens to fail on very slow machines like Github actions.

Pre-review checklist

• [x] I have split my patch into logically separate commits.
• [x] All commit messages clearly explain what they change and why.
• [x] PR description sums up the changes and reasons why they should be introduced.
• [ ] I have enabled appropriate tests in .github/workflows/build.yaml in gtest_filter.

Increased the acceptable time difference between sequential inserts in BasicsTests.Timestamps test, cause it happens to fail on very slow machines like Github actions.

[avelanarius]

I will re-run it a couple of times before merging.

[avelanarius]

As evidenced by https://github.com/scylladb/cpp-rust-driver/actions/runs/3129242901/jobs/5078086621, 150000 was still not enough. For the time being, I'll restart the integration test - this is not a priority thing to fix right now.