RFR: 8306083: Text.hitTest is incorrect when Text node is present in TextFlow

[Karthik P K]

The text run selected in `PrismTextLayout::getHitInfo()` method for character index calculation was not correct when Text node was embedded in TextFlow. Hence wrong character index value was calculated for the same.

Since only x, y coordinates were available in the above mentioned method, sending the text as a parameter to this method is necessary so as to know if the text run selected for character index calculation is correct. Along with this change modified the `PrismTextLayout::getHitInfo()` method to calculate the correct character index.

Added tests to validate the changes.

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Commit messages:
 - Fix character index calculation issue in Text node embedded in TextFlow

Changes: https://git.openjdk.org/jfx/pull/1157/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1157&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306083
  Stats: 372 lines in 7 files changed: 358 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/1157.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1157/head:pull/1157

PR: https://git.openjdk.org/jfx/pull/1157