Fix some bugs in the regression tests and add more regression tests (#295)

src/cmd/ksh93/tests/{basic.sh,builtins.sh,shtests}:
- Redirect error output from the ulimit builtin to silence irrelevant
  errors in the regression tests (these errors may occur when a
  command such as 'ulimit -t 4' is run before the regression tests).
- Shellquote the error messages from the getconf regression tests.

src/cmd/ksh93/tests/{arrays,io,variables}.sh:
- Backport the ksh2020 regression tests for the following bugs:
  https://github.com/att/ast/issues/23
  https://github.com/att/ast/issues/203
  https://github.com/att/ast/issues/472
  https://github.com/att/ast/issues/492
- Minor fix to POSIX mode regression tests in ksh93v-. In ksh93v-,
  [[ -o ?posix ]] doesn't return an error (because it's implemented
  in the bash mode). However, 'set -o posix' will fail in ksh93v-
  if it's not in bash compatibility mode, which causes this test
  script to exit prematurely.

src/cmd/ksh93/tests/{basic,pty}.sh:
- Add test for https://github.com/att/ast/issues/1461
- The ksh2020 fix for [ -t 1 ] in non-forking command substitutions
  caused the following bug in interactive shells:
    $ ( [ -t 1 ]; echo $? )
    1  # Always fails
  To avoid introducing this bug, this commit adds a regression
  test for it.

src/cmd/ksh93/tests/functions.sh:
- Add test for https://github.com/att/ast/issues/1160
  Put the test to the start of functions.sh (if it's at the end
  of the script, it refuses to fail under ksh2020). Output from
  this regression test when run against ksh2020:
    functions.sh[46]: eval'ing function dumps function body to
    stdout (got $' { eval "bar() { FAILURE; }"; }\n { FAILURE; }')

src/cmd/ksh93/tests/variables.sh

@@ -1344,5 +1344,14 @@ exp=5
 got=$($SHELL "$lineno_subshell")
 [[ $exp == $got ]] || err_exit "LINENO corrupted after leaving virtual subshell (expected $exp, got $got)"
 
+# ======
+# Check if ${.sh.file} is set to correct value after sourcing a file
+# https://github.com/att/ast/issues/472
+cat > $tmp/foo.sh <<EOF
+echo "foo"
+EOF
+. $tmp/foo.sh > /dev/null
+[[ ${.sh.file} == $0 ]] || err_exit "\${.sh.file} is not set to the correct value after sourcing a file"
+
 # ======
 exit $((Errors<125?Errors:125))