[NFC][libc++][test] Move time tests.

In the C++20 Standard time is no longer section under utilities, but
became its own chapter. This moves the time tests accordingly so their
location matches the

[NFC][libc++][test] Move time tests.

In the C++20 Standard time is no longer section under utilities, but
became its own chapter. This moves the time tests accordingly so their
location matches the current Standard.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D122745

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