[libc++][test] Add `void` cast to a discarded call to `remquo` (#213849)

For calls to `remquo`, the return values are generally expected to be
used, while it is rare to only use the value written via the pointer
parameter. Moreover, some implementors already planned to apply
`[[nodiscard]]` to `remquo`, see also https://llvm.org/PR171763. As a
result, it is perhaps better to add `void` cast to the discarded call to
`remquo` in the test file.

GitOrigin-RevId: d28193fa1ff64027cc5547355e9d70785268aec8