RFR: 8375683: Add notes for sctp tests [v3]

[SendaoYan]

> Hi all,
> 
> The sctp tests need libsctp.so to be installed, otherwise those tests will report skipped. Seems that there is no help information to descript how to run those tests. I think it's meaningful to add help information how to run those tests.

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

  Add index in testing.html

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29320/files
  - new: https://git.openjdk.org/jdk/pull/29320/files/0182358f..8f36333f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29320&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29320&range=01-02

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29320.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29320/head:pull/29320

PR: https://git.openjdk.org/jdk/pull/29320