[libcxx][test] fix allocator in allocator_propagation test

The converting constructor is ill-formed, and `==` is missing. (I didn't implement `!=` since the test is C++20-and-later only; I'll let the compiler do it for us.)

Drive-by: change 4-space indent on line 27 to 2-space indent to be consistent with the rest of the test.

Differential Revision: https://reviews.llvm.org/D131079