RFR: CODETOOLS-7902851: jcstress: Provide the ability to capture VM output

[Aleksey Shipilev]

In order to get PrintAssembly and other diagnostics from the forked VM, it would be instrumental for jcstress harness to capture the output from the forked VMs.

Example:

      [OK] o.o.j.t.fences.UnfencedDekkerTest
    (fork: #1, JVM args: [-Xlog:gc])
  Observed state   Occurrences              Expectation  Interpretation                                              
            0, 0       501,152   ACCEPTABLE_INTERESTING  Acceptable with no sequential consistency enforced          
            0, 1   137,897,148               ACCEPTABLE  Acceptable under sequential consistency                     
            1, 0   134,406,572               ACCEPTABLE  Acceptable under sequential consistency                     
            1, 1           229               ACCEPTABLE  Acceptable under sequential consistency                     

    VM output stream: 
        [0.003s][info][gc] Using G1
        [0.231s][info][gc] GC(0) Pause Young (Normal) (G1 Evacuation Pause) 100M->9M(2012M) 19.354ms
        [2.351s][info][gc] GC(1) Pause Young (Normal) (G1 Evacuation Pause) 106M->2M(2012M) 2.302ms

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Commit messages:
 - Capture VM output

Changes: https://git.openjdk.java.net/jcstress/pull/13/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=13&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902851
  Stats: 189 lines in 7 files changed: 121 ins; 28 del; 40 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/13.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/13/head:pull/13

PR: https://git.openjdk.java.net/jcstress/pull/13