Add more regression tests, mostly from ksh93v- and ksh2020

[JohnoKing]

List of added regression tests:

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: att#1452
- The trap builtin shouldn't segfault after receiving an invalid signal name. ksh2020 regression: att#1403
- Add a test to make sure invalid flags don't crash ksh. ksh2020 regression: att#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: att#1402
- The chmod builtin should modify the permissions of all files passed to it. ksh2020 regression: att#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: att#1409

src/cmd/ksh93/tests/exit.sh:
- Add regression tests from ksh2020 for the exit builtin: att@d9491d4

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 corrupts $foo in ksh93u+, ksh93v- and ksh2020:

  $ typeset -s foo=30000
  $ echo $foo
  5#1430000

This bug was fixed in commit 88a6baa, 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}: att#1092

[McDutchie]

Thanks, this will be very useful to avoid backporting these bugs in future. There's just one little thing to fix...