testsuite: Fix scan-lang-dump-not

[Nathan Sidwell]

pushed as obvious

turned out scan-lang-dump-not was broken in the 3 argument case -- I'd 
missed a necessary empty arg.  Fixed thusly.

             gcc/testsuite/
             * lib/scanlang.exp (scan-lang-dump-not): Fix 3-arg case.

-- 
Nathan Sidwell
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