bin/shtests: fix SHELL=/path/to/ksh assignment argument

Passing a SHELL=/path/to/ksh assignment-argument after the
bin/shtest command (as documented in 'shtest --man') made the
wrapper script produce inconsistent results: it would launch and
claim to test the default shell, but src/cmd/ksh93/tests/shtests
would actually test the shell indicated in the argument.

bin/shtests:
- Scan arguments for a SHELL= (or KSH=) assignment-argument,
  setting $KSH (and then exporting $SHELL) based on that if found.

(cherry picked from commit 1e78fbbd094dfc84989ecadf06231c6515fb7412)

bin/shtests

@@ -7,7 +7,20 @@
 # Brief help: bin/shtests --help
 #
 # By default, this runs your compiled arch/*/bin/ksh.
-# To use another ksh, run:  KSH=path/to/ksh bin/shtests
+#
+# Note: The test suite actually uses $SHELL to indicate the shell to test. But
+# we cannot use the $SHELL environment value on entry to this wrapper script,
+# as that is already used for the user's default login shell on most systems.
+
+# Process and remove any assignment-argument indicating the shell to test
+for arg do
+	case $arg in
+	( SHELL=* | KSH=* )
+		KSH=${arg#*=} ;;
+	( * )	set -- "$@" "$1" ;;
+	esac
+	shift
+done
 
 # Find root dir of ksh source
 mydir=$(dirname "$0") \