test: use np.nan for numpy >= 2.0.0

[Linchin]

Starting with numpy==2.0.0, np.NaN is replaced by np.nan. This is discovered by flakybot and this PR ensures both older and newer versions of numpy are supported.

Fixes #267 🦕

[parthea]

@Linchin Please could you check the coverage gap?

[Linchin]

@parthea Thanks, this was caused by prerelease tests not being included in coverage test results. From our discussion offline, I merged the unittest-prerelease.yml workflow into unittest.yml and made it one of coverage test's dependencies. This should close the coverage gap.

Note: As unittest.yml is synced by owlbot, I have to exclude unittest.yml from owlbot updates in the future. We are considering make synthtool updates via owlbot manual in order to reduce number of PRs in our repos. For more info, see internal b/330773846.