Add a test to ensure no zombie process left after the pcntl_waitpid call

[Atry]

Differential Revision: D34493604

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D34493604

[Atry]

The test added in this PR would fail in execve_execvpe_lightprocess.php and succeed in execve_execvpe.php.
The result difference indicates the preexisting issue of pid leak in the current LightProcess implementation.

$ hphp/test/run hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php
Running 1 tests in 1 threads (0 in serial)

FAILED: hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php
011+  741841 ?        Z      0:00 [sh] <defunct>

1 tests failed
(╯°□°）╯︵ ┻━┻

Run hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php by hand:
./buck-out/gen/hphp/hhvm/hhvm/hhvm -c /data/users/atry/fbsource/fbcode/hphp/test/slow/config.ini -vEval.EnableArgsInBacktraces=false -vEval.EnableIntrinsicsExtension=true -vEval.HHIRInliningIgnoreHints=false -vEval.HHIRAlwaysInterpIgnoreHint=false -vEval.FoldLazyClassKeys=false -vUnitFileCache.Path=/tmp/hphp-test-neovyS/unit-cache.sql -vEval.Jit=true  -vEval.ProfileHWEnable=false -vEval.EmbeddedDataExtractPath='/data/users/atry/fbsource/fbcode/buck-out/gen/hphp/hhvm/hhvm/hhvm_%{type}_%{buildid}' -vEval.JitWorkerThreads=1 -vEval.JitWorkerThreadsForSerdes=1  -vAutoload.DB.Path='/tmp/hphp-test-neovyS/hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php.autoloadDB.0' -vResourceLimit.CoreFileSize=0   -vServer.LightProcessFilePrefix=/tmp/lightprocess -vServer.LightProcessCount=5   --file 'hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php'

See failed test output and expectations:
cat /tmp/hphp-test-neovyS/hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php.diff
cat /tmp/hphp-test-neovyS/hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php.out
cat hphp/test/slow/ext_hsl/execve_execvpe_lightprocess.php.expectf

For xargs, list of failures is available using:
cat /tmp/hphp-test-neovyS/test-failures

Re-run just the failing tests:
hphp/test/run  $(cat /tmp/hphp-test-neovyS/test-failures)

Total time for all executed tests as run: 2.64s
Total time for all executed tests if run serially: 2.60s

$ hphp/test/run hphp/test/slow/ext_hsl/execve_execvpe.php
Running 1 tests in 1 threads (0 in serial)

All tests passed.
              |    |    |
             )_)  )_)  )_)
            )___))___))___)\
           )____)____)_____)\
         _____|____|____|____\\__
---------\      SHIP IT      /---------
  ^^^^^ ^^^^^^^^^^^^^^^^^^^^^
    ^^^^      ^^^^     ^^^    ^^
         ^^^^      ^^^

Total time for all executed tests as run: 2.44s
Total time for all executed tests if run serially: 2.40s