tests: add a new test for testing changing or clearing topics test-topic.t is getting bigger and bigger. This patch introduces a new test which includes test for clearing and changing topics. Since the tests are moved to a new file, this patch, removes those tests from test-topic.t also.

. $TESTDIR/testlib/pythonpath.sh

cat >> $HGRCPATH << EOF
[extensions]
docgraph=
EOF