shtests --posix: ensure consistent locale: unset all LC_* vars

When running tests in --posix/-p mode, not all LC_* variables were
unset, so that certain aspects of the locale could be non-POSIX.

src/cmd/ksh93/tests/shtests

@@ -244,8 +244,7 @@ then	utf8=0
 	if	! let 1.0 2>/dev/null
 	then	export LC_NUMERIC=C
 	fi
-else	unset LANG LC_ALL
-	export LC_NUMERIC=C
+else	unset LANG ${!LC_*}
 fi
 if	[[ $VMALLOC_OPTIONS ]]
 then	vmalloc_options=$VMALLOC_OPTIONS