RFR: 8294880: Review running time of jdk/internal/shellsupport/doc/JavadocHelperTest.java [v2]

[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.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  Move FullJavaDocHelperTest.java into its own group

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18837/files
  - new: https://git.openjdk.org/jdk/pull/18837/files/340a3b4f..7a45c94d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18837&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18837&range=00-01

  Stats: 9 lines in 1 file changed: 5 ins; 1 del; 3 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