[jdk11u-dev] RFR: 8268555: Update HttpClient tests that use ITestContext to jtreg 6+1

[Goetz Lindenmaier]

I downport this for parity with 11.0.13-oracle.

Response204V2Test.java: resolve imports due to context.
ShortResponseBody.java: resolve copyright and imports.

Change to test/jdk/java/net/httpclient/AggregateRequestBodyTest.java omitted.
The test is not in 11, it only came with "8252374 Add a new factory 
method to concatenate..." in 16.

Change to test/jdk/java/net/httpclient/CancelRequestTest.java omitted.
The test is not in 11, it only came with "8245462: HttpClient send throws InterruptedException when interrupted but does not cancel request" in 16.

Change to test/jdk/java/net/httpclient/ISO_8859_1_Test.java b/test/jdk/java/net/httpclient/ISO_8859_1_Test.java omitted.
The test is not in 11, it only came with "8255244 HttpClient: Response headers contain incorrectly encoded Unicode characters" in 16.

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Commit messages:
 - Backport da043e99b830fa4fcbfdbdbed182abc394ba6fb1

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/448/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=448&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268555
  Stats: 214 lines in 7 files changed: 195 ins; 0 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/448.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/448/head:pull/448

PR: https://git.openjdk.java.net/jdk11u-dev/pull/448