Tests: portability improvements and additional test (#40)

* Add test for :- substitution w/ empty alternative

The correct behaviour here is to substitute the empty string, but
osh currently substitutes nothing.

* Use /usr/bin/env time rather than /usr/bin/time

This is more portable to operating systems such as NixOS which do
not have /usr as a global shared namespace.

* spec.sh: Find busybox portably

spec/show-fd-table.sh

@@ -13,7 +13,7 @@ show-fd-table() {
  echo "   function in pid $$"
  # Make it truly parallel
  sleep 0.5
  /usr/bin/time --output /tmp/$$.txt -- spec/bin/show_fd_table.py "$@"
  /usr/bin/env time --output /tmp/$$.txt -- spec/bin/show_fd_table.py "$@"
}

# Trying to recreate spec-runner problem with file descriptors.
@@ -25,7 +25,7 @@ main() {
  echo

  # File descriptor 3 is open!
  /usr/bin/time --output /tmp/task.txt -- spec/bin/show_fd_table.py 
  /usr/bin/env time --output /tmp/task.txt -- spec/bin/show_fd_table.py
  echo

  # Cannot reproduce problem.  What's the deal with descriptors 8 and 9?  Oh

spec/var-sub-quote.test.sh

@@ -14,6 +14,11 @@
# LexState.OUTER.  If we have "${}", then it's parsed as LexState.DQ.  That
# makes sense I guess.  Vim's syntax highlighting is throwing me off.

### :- with empty alternative
foo=
argv.py "${foo:-}"
# stdout: ['']

### :-
empty=''
argv.py ${empty:-a} ${Unset:-b}

test/spec-runner.sh

@@ -62,7 +62,7 @@ run-task-with-status() {
  shift

  # --quiet suppresses a warning message
  /usr/bin/time \
  /usr/bin/env time \
    --output $out_file \
    --format '%x %e' \
    -- "$@" || true  # suppress failure

test/spec.sh

@@ -37,7 +37,7 @@ readonly REF_SHELLS=($DASH $BASH $MKSH)
install-shells() {
  sudo apt-get install busybox-static mksh zsh
  mkdir -p _tmp/shells
  ln -s -f --verbose /bin/busybox $BUSYBOX_ASH
  ln -s -f --verbose "$(which busybox)" $BUSYBOX_ASH
}

# TODO: Maybe do this before running all tests.