RFR: 8338469: com/sun/jdi/DataDumpTest.java failed with Not a debuggee, or not listening for debugger to attach

[Chris Plummer]

There are issues with the test attaching to the debuggee too soon, and the debug agent isn't ready. yet. This test is based on ProcessAttachTest, which does not have this issue. I eventually realized the reason why is because ProcessAttachTest has this little snippet of code, which I had removed from DataDumpTest:

             // Wait for the process to start
            InputStream is = p.getInputStream();
            is.read(); 

This is waiting for the start of the debug agent's "Listening..." message. I've re-added this to DataDumpTest, and it seems to fix the issue. Tested by running 20 times with `-Xcomp` on all supported platforms with no failures. It used to fail about 2/3 of the time.

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Commit messages:
 - Wait until first character can be read before trying to attach to debuggee process.

Changes: https://git.openjdk.org/jdk/pull/20604/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20604&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338469
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20604.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20604/head:pull/20604

PR: https://git.openjdk.org/jdk/pull/20604