RFR: 8257511: JDK-8254082 brings regression to AbstractStringBuilder.insert(int dstOffset, CharSequence s, int start, int end)

[Claes Redestad]

This patch fixes a place where we failed to adjust properly for getBytes taking a length rather than end offset. (Something the public API in AbstractStringBuilder is apparently inconsistent about..)

Although this was caught by JCK testing, this patch adds a few trivial sanity tests to get at least some sanity checking into tier1

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Commit messages:
 - Adjust for inconsistent API..
 - JDK-8254082 brings regression to AbstractStringBuilder.insert(int dstOffset, CharSequence s, int start, int end)

Changes: https://git.openjdk.java.net/jdk/pull/1541/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1541&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257511
  Stats: 25 lines in 2 files changed: 16 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1541.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1541/head:pull/1541

PR: https://git.openjdk.java.net/jdk/pull/1541