[libc++][test] Refactor increasing_allocator (#115671)

The increasing_allocator<T> class, originally introduced to test shrink_to_fit()
for std::vector, std::vector<bool>, and std::basic_string, ha

[libc++][test] Refactor increasing_allocator (#115671)

The increasing_allocator<T> class, originally introduced to test shrink_to_fit()
for std::vector, std::vector<bool>, and std::basic_string, has duplicated
definitions across several test files. Given the potential utility of this
class for capacity-related tests in various sequence containers, this patch
refactors the definition of increasing_allocator<T> into a single, reusable
location.

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