[libc++][test] Improve tests for assign in std::vector and vector<bool> (#119163)

This PR enhances the test coverage for std::vector::assign by adding new
tests for several important test cases tha

[libc++][test] Improve tests for assign in std::vector and vector<bool> (#119163)

This PR enhances the test coverage for std::vector::assign by adding new
tests for several important test cases that were previously missing, as
shown in the following table:

| test cases | forward_iterator | input_iterator |
|-----------------------------------|------------------|----------------|
| new_size > capacity() | Yes | Yes |
| size() < new_size <= capacity() | No | No |
| new_size <= size() | No | No |

Similarly, no tests have previously covered `assign(InputIterator, InputIterator)`
and `assign(size_type, const value_type&)` for `vector<bool>`.

With this patch applied, all missing tests are covered.

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