shtests: don't require specifying .sh extension for test script

It was annoying me that I always had to type out the .sh in
'shtests builtins.sh', etc. to execute a specific regression test
script. All the test scripts end in .sh, so this is superflous.

src/cmd/ksh93/tests/shtests:
- Add .sh to test script arguments if not already present.

(cherry picked from commit d780e701fbf6f27b6e79f88542a3175bf5dd85e9)

src/cmd/ksh93/tests/shtests

@@ -308,7 +308,8 @@ then	if	[[ -x $SHELL-g ]]
 fi
 typeset -A tests
 for i in ${*-*.sh}
-do	if	[[ ! -r $i ]]
+do	[[ $i == *.sh ]] || i+='.sh'
+	if	[[ ! -r $i ]]
 	then	echo $0: $i: not found >&2
 		continue
 	fi