- #3066
- reduce `TestServer` -> `PhysicalFileProvider` -> `FileSystemWatcher` count enough to run with Core CLR on Linux
- remove use of `HttpClient.DefaultRequestHeaders`; any client change affects other tests
- remove use of `RequestBuilder` class; creates a per-test `HttpClient` and requires the `TestServer`
- updated a few expectations because `CommonTestEncoder` does JavaScript a bit differently
- "JavaScriptEncode[[...]]" -> "JavaScriptStringEncode[[...]]"
- side benefit: xUnit reports functional tests execute for only ~12.4s; was >30s before this change
Infrastructure: Enhance `MvcTestFixture`
- handle `ConfigureServices()` methods that are not `void`
- handle `Configure(IApplicationBuilder, ILoggerFactory)`
- ensure server is initialized with consistent `CurrentCulture` and `CurrentUICulture`
- add `FilteredDefaultAssemblyProviderFixture<TStartup>` and `MvcEncodedTestFixture<TStartup>`
- add `MvcTextFixture.AddAdditionalServices()` extension point supporting these
- do not expose the `TestServer`; an anti-pattern for tests to manipulate the server
- update class names to match containing files
- use existing `TestApplicationEnvironment`
- apply some `MvcTestFixture` improvements to the shared `TestApplicationEnvironment` class
- remove unused methods from `TestHelper`
nits:
- touched-up some leftover `_app` &c declarations to be more explicit and minimize `using`s
- moved statements into correct sections of methods in `RoutingTests`
- removed `TestLoggerFactory` and related classes from `TagHelperSampleTest`
