Separate HostResolverManagerTest

host_resolver_manager_unittest.cc contains 15k+ LoC. It takes
considerable amount of time to compile and clangd is really slow on
the file. To make adding tests easier and less painful, this CL exposes
HostResolverManager{,Dns}Test so that new tests can live in separate
files. As an example this CL moves
HostResolverManagerIPv6ReachabilityOverrideTest into a separate file.

CreateValidDnsConfig() and MockHostResolverProc are moved into
dns_test_util.h as these will be shared among tests.

Bug: 41493696

Change-Id: I402296c080e584e738fea9fef9e7d4e882ff59f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5359593
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270812}