Misc regression test fixes

src/cmd/ksh93/tests/basic.sh:
- Fix syntax error (unbalanced single quote) in two -c script
  invocations. It only failed to throw a syntax error due to a
  problematic hack in ksh that may be removed soon.
  See: https://github.com/ksh93/ksh/issues/199

src/cmd/ksh93/tests/builtins.sh,
src/cmd/ksh93/tests/io.sh:
- Redirect standard error on two ksh -i invocations to /dev/null
  to work around the test hanging on AIX.

src/cmd/ksh93/tests/comvario.sh:
- Remove duplicate copyright header.
- Fix warning format.

src/cmd/ksh93/tests/functions.sh:
- Fix the 'TERM signal sent to last process of function kills the
  script' test so that it works on AIX. We cannot rely on grepping
  'ps' output as the external 'sleep' command does not show the
  command name on AIX. Instead, find it by its parent PID.

src/cmd/ksh93/tests/locale.sh,
src/cmd/ksh93/tests/substring.sh:
- Rewrite the very broken multibyte locale tests (two outright
  syntax errors due to unbalanced quotes, and none of the tests
  actually worked).
- Since they set LC_ALL, move them to locale.sh.

src/cmd/ksh93/tests/variables.sh:
- Redirect stderr on some 'ulimit -t unlimited' invocations (which
  fork subshells as the intended side effect) to /dev/null in case
  that exceeds a system-defined limit.

src/cmd/ksh93/tests/variables.sh

@@ -710,7 +710,7 @@ actual=$(
 		(
 			echo ${.sh.subshell} | cat	# left element of pipe should increase ${.sh.subshell}
 			echo ${.sh.subshell}
-			ulimit -t unlimited		# fork
+			ulimit -t unlimited 2>/dev/null	# fork
 			echo ${.sh.subshell}		# should be same after forking existing virtual subshell
 		)
 		echo ${.sh.subshell}			# a background job should also increase ${.sh.subshell}
@@ -730,7 +730,7 @@ set --
 
 unset r v x
 (
-	ulimit -t unlimited  # TODO: this test messes up LINENO past the subshell unless we fork it
+	ulimit -t unlimited 2>/dev/null  # TODO: this test messes up LINENO past the subshell unless we fork it
 	x=foo
 	for v in EDITOR VISUAL OPTIND CDPATH FPATH PATH ENV RANDOM SECONDS _ LINENO
 	do	nameref r=$v
@@ -1082,7 +1082,7 @@ $SHELL -c '
 # ======
 # ${.sh.pid} should be the forked subshell's PID
 (
-	ulimit -t unlimited
+	ulimit -t unlimited 2>/dev/null  # fork the subshell
 	[[ ${.sh.pid} == $$ ]]
 ) && err_exit "\${.sh.pid} is the same as \$$ (both are $$)"