<i18n dev> RFR: 8335668: NumberFormat integer only parsing should throw exception for edge case

[Justin Lu]

Please review this PR which corrects a case in NumberFormat integer only parsing.

[JDK-8333755](https://bugs.openjdk.org/browse/JDK-8333755) fixed integer only parsing when the value has a suffix, although it caused incorrect behavior for the following case: when the parsed string does not contain an integer portion, and the format has integer only parsing, parsing should fail, instead of 0 being returned. For example, 


var fmt = NumberFormat.getIntegerInstance();
fmt.parse(".5", new ParsePosition(0)); // should return null, not 0


The changes to the _badParseStrings_ data provider in _StrictParseTest.java_ are needed since those cases _should_ fail in different indexes depending on if integer parsing is enabled. Thus, they were updated to ensure they fail for both integer and non-integer parsing with the same errorIndex.

In the fix itself, I also updated the initial value of `intIndex` to -1 from 0, to provide better clarity.

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Commit messages:
 - clean up + add more general test cases
 - init

Changes: https://git.openjdk.org/jdk/pull/20101/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335668
  Stats: 65 lines in 3 files changed: 54 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/20101.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20101/head:pull/20101

PR: https://git.openjdk.org/jdk/pull/20101