Add tests for `yearMonthDayToTimestampInSeconds` utility

[miorel]

Add a file workspaces/util/src/__tests__/yearMonthDayToTimestampInSeconds-test.ts, for testing the yearMonthDayToTimestampInSeconds utility.

To see some PRs that add tests, take a look at #288 and #290 for inspiration.

[lohithasiripurapu]

Hi, I'd like to work on this issue, can you assign it to me

[yulianka]

Hi, I'd like to work on this issue, can you assign it to me

Done. And good luck!

[miorel]

@lohithasiripurapu, please let us know if you need help getting started on this!

[miorel]

Releasing the issue due to inactivity, @lohithasiripurapu let us know if you end up coming back to this.