RFR: 8294880: Review running time of jdk/internal/shellsupport/doc/JavadocHelperTest.java

[Hannes Wallnöfer]

Please review a change to move a long-running shellsupport test from langtools tier1 to tier2, while modifying the tier1 test to only retrieve doc comments for members of `java.lang.StringBuilder`, which is where [the original bug](https://bugs.openjdk.org/browse/JDK-8189778) occurred.  This reduces the running time in the tier1 test from around 90 seconds to 2 seconds on my computer. The tier1 test also adds a non-null check for retrieved doc comments.

I used the existing `langtools_jshell_unstable` test group to add the new test in langtools tier2, which is not a perfect match for the test, but close enough and helps to keep langtools `TEST.groups` file simple.

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Commit messages:
 - JDK-8294880: Review running time of jdk/internal/shellsupport/doc/JavadocHelperTest.java

Changes: https://git.openjdk.org/jdk/pull/18837/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18837&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294880
  Stats: 157 lines in 3 files changed: 126 ins; 19 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/18837.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18837/head:pull/18837

PR: https://git.openjdk.org/jdk/pull/18837