Exclude some warnings from the doxygen test

Apparently travis has an old version of doxygen that doesn't know all tags in
our config. That's not something we care about, we only want to know about
warnings in our doxygen content

tests/scripts/doxygen.sh

@@ -16,10 +16,14 @@ if make apidoc > doc.out 2>doc.err; then :; else
     exit 1;
 fi
 
-if grep warning doc.out doc.err; then
+cat doc.out doc.err | \
+    grep -v "warning: ignoring unsupported tag" \
+    > doc.filtered
+
+if grep "warning" doc.filtered; then
     echo "FAIL" >&2
     exit 1;
 fi
 
 make apidoc_clean
-rm -f doc.out doc.err
+rm -f doc.out doc.err doc.filtered