Regress tests: keep common code in one place

src/cmd/ksh93/tests/_common:
- Added. This keeps one common version of 'err_exit', 'warning',
  and other init code.

src/cmd/ksh93/tests/*.sh:
- Source _common as a dot script.
- Remove 50-odd, occasionally slightly different, versions of the
  common code.
- Some minor tweaks.

src/cmd/ksh93/tests/_common

@@ -0,0 +1,31 @@
+########################################################################
+#                                                                      #
+#              This file is part of the ksh 93u+m package              #
+#          Copyright (c) 1982-2012 AT&T Intellectual Property          #
+#             Copyright (c) 2021 Contributors to ksh 93u+m             #
+#                    <https://github.com/ksh93/ksh>                    #
+#                      and is licensed under the                       #
+#                 Eclipse Public License, Version 1.0                  #
+#                                                                      #
+#                A copy of the License is available at                 #
+#          http://www.eclipse.org/org/documents/epl-v10.html           #
+#         (with md5 checksum b35adb5213ca9657e911e9befb180842)         #
+#                                                                      #
+#                  Martijn Dekker <martijn@inlv.org>                   #
+#                                                                      #
+########################################################################
+
+_message()
+{
+	print -r $'\t'"${Command}[$1]: ${@:2}" >&2
+}
+alias err_exit='_message "$((Errors++,LINENO))"'   # inaccurate err_exit name kept for historical integrity :)
+alias warning='_message "$LINENO" "warning:"'
+
+Command=${0##*/}
+integer Errors=0
+
+if	! [[ -d $tmp && -w $tmp && $tmp == "$PWD" ]]
+then	print -r "$Command: \$tmp not set; run this from shtests. Aborting." >&2
+	exit 1
+fi