Add more regression tests, mostly from ksh93v- and ksh2020 (#216)

src/cmd/ksh93/tests/arrays.sh,
src/cmd/ksh93/tests/arrays2.sh:
- Backport some regression tests from ksh93v- for associative
  arrays.

src/cmd/ksh93/tests/basic.sh:
- Add ksh93v- regression tests for background process output in
  backtick and shared-state command substitutions as well as
  functions used in command substitutions.

- Add regression tests for using EXIT traps in subshells. In
  ksh93v- and ksh2020 EXIT traps don't work in forked subshells:
  https://github.com/att/ast/issues/1452
- The trap builtin shouldn't segfault after receiving an invalid
  signal name. ksh2020 regression:
  https://github.com/att/ast/issues/1403
- Add a test to make sure invalid flags don't crash ksh.
  ksh2020 regression: https://github.com/att/ast/issues/1284
- Test for an illegal seek error when using the 'join' command with
  process substitutions. ksh93v- regression:
  https://www.mail-archive.com/ast-users@lists.research.att.com/msg00816.html

src/cmd/ksh93/tests/bracket.sh:
- Add some regression tests from ksh93v- for the -eq test operator.

src/cmd/ksh93/tests/builtins.sh:
- Move the regression test for 'exit' in an interactive shell to
  the exit.sh script.
- Test for assignments preceding the command builtin persisting
  after an error. ksh2020 regression:
  https://github.com/att/ast/issues/1402
- The chmod builtin should modify the permissions of all files
  passed to it. ksh2020 regression:
  https://github.com/att/ast/issues/949
- Add regression tests for the cd builtin. In ksh93v- 2013-10-10
  alpha, using cd on a directory without an execute bit doesn't
  cause an error. The test for using cd on a normal file was
  backported from ksh93v-.
- Backport a ksh93v- regression test for the exit status
  from 'kill %'.

src/cmd/ksh93/tests/case.sh:
- Test for a segfault when ksh handles an invalid character class
  in a pattern. ksh2020 regression:
  https://github.com/att/ast/issues/1409

src/cmd/ksh93/tests/exit.sh:
- Add regression tests from ksh2020 for the 'exit' builtin:
  d9491d46

src/cmd/ksh93/tests/io.sh:
- Add a regression test from ksh93v- for a process substitution
  hang. This test fails in the 93v- 2013 alpha but succeeds in
  the 2014 beta.

src/cmd/ksh93/tests/math.sh:
- 'typeset -s foo=30000' adds garbage to $foo in ksh93u+, ksh93v-
  and ksh2020:
  $ typeset -s foo=30000
  $ echo $foo
  5#1430000
  This bug was fixed in commit 88a6baa1, but that commit didn't
  add a regression test for it.

src/cmd/ksh93/tests/variables.sh:
- Add a regression test for $PS4 incorrectly unsetting
  ${.sh.subshell}: https://github.com/att/ast/issues/1092

src/cmd/ksh93/tests/variables.sh

@@ -1258,5 +1258,13 @@ exp='foo/foobar/fool'
 got=$(IFS=/; foo=bar foobar=fbar fool=pity; print -r -- "${!foo*}")
 [[ $got == "$exp" ]] || err_exit "\${!foo*}: expected $(printf %q "$exp"), got $(printf %q "$got")"
 
+# ======
+# In ksh93v- ${.sh.subshell} is unset by the $PS4 prompt
+# https://github.com/att/ast/issues/1092
+exp='0'
+got="$($SHELL -c 'PS4="${.sh.subshell}"; echo ${.sh.subshell}')"
+[[ "$exp" == "$got" ]] || err_exit "\${.sh.subshell} is wrongly unset in the \$PS4 prompt" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))