### What changes were proposed in this pull request?
[SPARK-27122](https://github.com/apache/spark/pull/24088) fixes `ClassCastException` at `yarn` module by introducing `DelegatingServletContextHandler`. Initially, this was discovered with JDK9+, but the class path issues affected JDK8 environment, too. After [SPARK-28709](https://github.com/apache/spark/pull/25439), I also hit the similar issue at `streaming` module.
This PR aims to fix `streaming` module by adding `getContextPath` to `DelegatingServletContextHandler` and using it.
### Why are the changes needed?
Currently, when we test `streaming` module independently, it fails like the following.
```
$ build/mvn test -pl streaming
...
UISeleniumSuite:
- attaching and detaching a Streaming tab *** FAILED ***
java.lang.ClassCastException: org.sparkproject.jetty.servlet.ServletContextHandler cannot be cast to org.eclipse.jetty.servlet.ServletContextHandler
...
Tests: succeeded 337, failed 1, canceled 0, ignored 1, pending 0
*** 1 TEST FAILED ***
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
```
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
Pass the Jenkins with the modified tests. And do the following manually.
Since you can observe this when you run `streaming` module test only (instead of running all), you need to install the changed `core` module and use it.
```
$ java -version
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_222-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.222-b10, mixed mode)
$ build/mvn install -DskipTests
$ build/mvn test -pl streaming
```
Closes#25791 from dongjoon-hyun/SPARK-29087.
Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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