Fix portability problem with /usr/bin/awk on Solaris 10 in stats test

The /usr/bin/awk on Solaris 10 is a pre-POSIX Awk that, among other
limitations, does not recognize the ARGV array as special.

ChangeLog

@@ -1,5 +1,9 @@
 2021-05-20  Jacob Bachmeyer  <jcb@gnu.org>
 
+	* testsuite/runtest.main/nested/testsuite/stat.test/unit-sub.exp:
+	Explicitly pass /dev/null to unit-sub.awk because /usr/bin/awk on
+	Solaris 10 does not seem to recognize assignment to ARGV.
+
 	* dejagnu: Search for a POSIX Awk and validate that at least a
 	simple Awk program actually works.
 	* testsuite/launcher.all/command.exp: Add tests for error produced

testsuite/runtest.main/nested/testsuite/stat.test/unit-sub.exp

@@ -22,5 +22,7 @@ load_lib dejagnu.exp
 
 if { [info exists STATS_TEST] } {
     host_execute [which awk] -f [testsuite file -source -test unit-sub.awk] \
-	UNIT_RESULT=$STATS_TEST
+	UNIT_RESULT=$STATS_TEST /dev/null
 }
+# Solaris 10 /usr/bin/awk needs /dev/null actually passed on the command
+# line, but will run unit-sub.awk without problems if that is done.