Many bug fixes, new tests, and test reorganization.

- Many bug fixes and code improvements for prefix ops like ${#a},
  suffix ops like ${a%suffix}, and test operations like ${a:-}.  Rewrite
  _EvalBracedVarSub to use value instead of part_value.  More consistent
  undefined handling, etc.

- Partial bug fix for $? of pipelines.  This uncovered a
  nondeterministic bug in $PIPESTATUS which still needs to be fixed.

- Various array bug fixes.

- Add new special-vars test for $?, $#, etc.

- Move array-decay to its own test, since it's a non-Oil feature.

core/cmd_exec.py

@@ -695,6 +695,7 @@ def _RunPipeline(self, node):

    # TODO: Set PipeStatus() in self.mem
    pipe_status = pi.Run()
    #log('pipe_status %s', pipe_status)

    if self.exec_opts.pipefail:
      # If any process failed, the status of the entire pipeline is 1.
@@ -734,15 +735,6 @@ def _Execute(self, node):
        p = Process(thunk, fd_state=self.fd_state, redirects=redirects)
        status = p.Run()

        if os.WIFEXITED(status):
          status = os.WEXITSTATUS(status)
          #print('exited with code', code)
        else:
          sig = os.WTERMSIG(status)
          #print('exited with signal', sig)
          # TODO: Is this right?
          status = 0

      else:  # Internal
        for r in redirects:
          r.ApplyInParent(self.fd_state)

core/process.py

@@ -14,7 +14,7 @@
import os
import sys

from core import util  # log
from core.util import log
from core.id_kind import REDIR_DEFAULT_FD


@@ -310,8 +310,8 @@ def RunInParent(self):
    try:
      os.execvpe(self.argv[0], self.argv, env)
    except OSError as e:
      util.log('Unexpected error in execvpe(%r, %r, ...): %s', self.argv[0],
               self.argv, e)
      log('Unexpected error in execvpe(%r, %r, ...): %s', self.argv[0],
          self.argv, e)
      # Command not found means 127.  TODO: Are there other cases?
      sys.exit(127)
    # no return
@@ -471,11 +471,18 @@ def Start(self):
    for r in self.redirects:  # here docs
      r.AfterForkInParent()

  # TODO: Should be a free function.  Not using self!
  def Wait(self):
    # NOTE: Need to check errors
    wait_pid, status = os.wait()

    # TODO: split up status?
    # TODO: change status in more cases.
    if os.WIFSIGNALED(status):
      pass
    elif os.WIFEXITED(status):
      status = os.WEXITSTATUS(status)
      #log('exit status: %s', status)

    return status

  def Run(self):
@@ -537,13 +544,17 @@ def Run(self):
    for p in self.procs:
      #print('start', p)
      p.Start()
      # TODO: Return pid

    pipe_status = []

    # TODO: Could do some sort of garbage collection here  too.

    for p in self.procs:
      #print('Wait', p)
      pipe_status.append(p.Wait())
      # BUG: This doesn't do things in order!  It just calls os.wait()!
      # Need to key off wait_pid

      status = p.Wait()
      #log('Process %s returned status %s', p, status)
      pipe_status.append(status)

    return pipe_status