Refactor tests and fix possible overflow in ICU case conversion

[Flamefire]

• Refactor tests and remove/combine/handle special cases for some ICU versions in tests

Prepares for adding better tests for basic_format

• Fix possible overflow in ICU (case) conversion
• Fix loss of precision when getting the time from a timepoint (conversion from double to posix_time to double)
• Avoid the bug in the time conversion resulting in always zero nanoseconds resulting from wrong conversion from seconds to nanoseconds

Extract the size type used by the ICU function from its signature and cast the parameters after checking the range.

[codecov[bot]]

Codecov Report

Merging #117 (3a24b94) into develop (7fb7ec5) will increase coverage by 0.21%. The diff coverage is 94.09%.

Additional details and impacted files

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