RFR: 8377675: java.net.http tests should not depend on ../../../com/sun/net/httpserver test classes

[Daniel Fuchs]

Some HttpClient tests - like `test/jdk/java/net/httpclient/SmokeTest.java`, depend on non-library test classes that were developed for testing the HttpServer, such as `../../../com/sun/net/httpserver/FileServerHandler.java` which requires compiling using `@compile`.

Instead of depending on HttpServer test classes, these tests should use functionalities from the common servers provided by the `HttpServerAdapters`, offered by the `test/jdk/java/net/httpclient/lib` library. An adapted version of the `../../../com/sun/net/httpserver/FileServerHandler.java` could be provided by the `HttpServerAdapters` helper.

There are also 5 implementations of stand-alone echo handlers in the httpclient test base. This change remove 4 of them, keep only the implementation in `HttpServerAdapters`, and updates all tests that were relying on the removed implementations to use that provided by `HttpServerAdapters`.

This is a test-only change.

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Commit messages:
 - 8377675 java.net.http tests should not depend on ../../../com/sun/net/httpserver test classes

Changes: https://git.openjdk.org/jdk/pull/29702/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29702&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377675
  Stats: 1149 lines in 17 files changed: 352 ins; 539 del; 258 mod
  Patch: https://git.openjdk.org/jdk/pull/29702.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29702/head:pull/29702

PR: https://git.openjdk.org/jdk/pull/29702