tests: port to Solaris diff

* tests/extrac13.at: Don't assume that "diff -c" outputs nothing
when there are no differences.  This is not true on Solaris,
where it outputs "No differences encounted".

tests/extrac13.at

@@ -33,20 +33,20 @@ echo target1 >target1
 
 tar -cf archive.tar -C src . &&
 tar -xf archive.tar -C dst1 --warning=no-timestamp &&
-diff -c src/file1 dst1/file1 &&
-diff -c target1 dst1/target1
+diff src/file1 dst1/file1 &&
+diff target1 dst1/target1
 
 ln -s target1 dst2/file1
 echo target1 >dst2/target1
 tar --overwrite -xf archive.tar -C dst2 --warning=no-timestamp &&
-diff -c src/file1 dst2/file1 &&
-diff -c target1 dst2/target1
+diff src/file1 dst2/file1 &&
+diff target1 dst2/target1
 
 ln -s target1 dst3/file1
 echo target1 >dst3/target1
 tar --overwrite -xhf archive.tar -C dst3 --warning=no-timestamp &&
-diff -c src/file1 dst3/file1 &&
-diff -c src/file1 dst3/target1
+diff src/file1 dst3/file1 &&
+diff src/file1 dst3/target1
 ],
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