<i18n dev> RFR: JDK-8325908: Finish removal of IntlTest and CollatorTest

[Justin Lu]

Please review this PR which fixes / finishes the rest of the IntlTest test framework removal in java.text and java.util.i18n tests.

For context, the IntlTest class only ran methods prefixed by _test_ or _Test_ with public visibility and was originally removed due to some tests spuriously passing that were not aware of the specific running requirements.

This PR includes the following changes,
- There were some tests with package-private visibility that were never ran by IntlTest. Those tests do appear to be valid tests, and thus have been updated with `@Test` annotation.
- The test method DFSExponenTest() in DFSExponential.java was not prefixed by test and thus never ran by IntlTest. It is a valid test and should be ran as well.
- DateFormatRoundTripTest.java was supposed to remain a non JUnit test, however the run invocation was converted to `@run junit ...` this has been switched back to `@run main ...`
- There were two instances of the script missing some tests that should have had their methods updated with the `@Test` annotation: APITest.java and DFSSerialization.java. These tests have been updated accordingly.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/17858/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17858&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325908
  Stats: 77 lines in 7 files changed: 46 ins; 11 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/17858.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17858/head:pull/17858

PR: https://git.openjdk.org/jdk/pull/17858