RFR: 8284077: Create an automated test for JDK-4170173

[Srinivas Mandalika]

Create an automated test for [JDK-4170173](https://bugs.openjdk.java.net/browse/JDK-4170173)

Issue
JTextComponent.AccessibleJTextComponent.getAfterIndex(int part, int index) works incorrectly, when 'part' parameter is AccessibleText.WORD.
It returns a space (" ") instead of the correct word.

The test verifies the fix for this behavior by checking the getAfterIndex for AccessibleText.CHARACTER,AccessibleText.WORD,AccessibleText.SENTENCE for the components JTextField, JTextArea, JEditorPane.

While working on this test case there was a related bug relevant to this [JDK-4170174](https://bugs.openjdk.java.net/browse/JDK-4170174) 
This is marked as duplicate, addressess a similar issue.
It indicates that JTextComponent.AccessibleJTextComponent.getBeforeIndex(int part, int index) works incorrectly, when part parameter is AccessibleText.WORD.
It returns a space (" ") instead of correct word.

Hence an additional test was added for this, for verifying the behavior of getBeforeIndex.
AccessibleText.CHARACTER,AccessibleText.WORD,AccessibleText.SENTENCE for the components JTextField, JTextArea, JEditorPane.

The two tests have multiple distinct assertions. For this reason, as well as for maintainability, the two were not clubbed into a single test.
However, the two tests are still similar in the functional flow of the code and the functionality they are testing as well - hence they have been clubbed into a single review.
This review is for migrating tests from a closed test suite to open.

Testing:
The tests ran successfully on Mach5 with multiple runs (30) on windows-x64, linux-x64 and macos-x64.

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Commit messages:
 - 8284077: Create an automated test for JDK-4170173
 - 8284077: Create an automated test for JDK-4170173

Changes: https://git.openjdk.java.net/jdk/pull/8138/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8138&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284077
  Stats: 247 lines in 2 files changed: 247 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8138.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8138/head:pull/8138

PR: https://git.openjdk.java.net/jdk/pull/8138