[BUG] Build RAFT tests with -Wall -Werror

[trxcllnt]

As a header-only library, warnings in RAFT can cause errors in consuming libs that compile with -Werror. Building tests with -Wall -Werror like RMM does should catch most problems before they're released.

For example, see this error in a RAFT header when compiling cuML:

raft/cpp/include/raft/sparse/hierarchy/detail/mst.cuh: In instantiation of 'void raft::hierarchy::detail::build_sorted_mst(const raft::handle_t&, const value_t*, const value_idx*, const value_idx*, const value_t*, size_t, size_t, rmm::device_uvector<value_idx>&, rmm::device_uvector<value_idx>&, rmm::device_uvector<value_t>&, size_t, raft::distance::DistanceType, int) [with value_idx = int; value_t = float; size_t = long unsigned int]':
raft/cpp/include/raft/sparse/hierarchy/single_linkage.hpp:80:47:   required from 'void raft::hierarchy::single_linkage(const raft::handle_t&, const value_t*, size_t, size_t, raft::distance::DistanceType, raft::hierarchy::linkage_output<value_idx, value_t>*, int, size_t) [with value_idx = int; value_t = float; raft::hierarchy::LinkageDistance dist_type = raft::hierarchy::PAIRWISE; size_t = long unsigned int]'
cuml/cpp/src/hierarchy/linkage.cu:32:117:   required from here
raft/cpp/include/raft/sparse/hierarchy/detail/mst.cuh:204:410: warning: format '%d' expects argument of type 'int', but argument 4 has type 'size_t' {aka 'long unsigned int'} [-Wformat=]

[trxcllnt]

I started this as a feature-request, but realized it's better as a bug. That's why the label is wrong.

[dantegd]

PR #187 adds the cmake side to enable this easily, was going to push it in the same PR but (as usual) once I fix what it looked like a single warning, more started popping up, so it probably needs to be a follow up.

[dantegd]

Closed nu #299