Adapt existing HttpClientMiniStress tests to support HTTP/2 (dotnet/corefx#38619)

Adapt existing HttpClientMiniStress tests to support HTTP/2 (dotnet/corefx#38619)

This adapts the existing HttpClientMiniStress tests to run with HTTP/2 as well as HTTP/1.1.  Some caveats, though:
- All of these existing tests spin up a new server per request, so they're not stressing arguably the most important aspect of the client's HTTP/2 implementation, that of sharing the single connection across all requests.  Our loopback server is not currently robust enough to handle concurrent requests; we should look at switching over to using Kestrel.
- The tests run really slowly, ~40x slower than for HTTP/1.1.  However, the HTTP/1.1 tests are non-SSL, these are of course SSL, and it looks like that's actually the difference.  In particular, something about how our test server gets its test certificate appears to be taking the vast majority of the time. (We should separately see if we could tweak that, not just for this purpose, but more so to help speed up the whole suite.)

Commit migrated from https://github.com/dotnet/corefx/commit/a7c893ab3d3e50b11c7fa2338882c8964e0a290d