InvalidSnapshotTest corrupts its own test data

InvalidSnapshotTest.testSnapshot starts an instance of ZooKeeperServer on the version-controlled invalidsnap directory, which, as a side-effect, "fixes" the following snapshot—which is broken on purpose (see ZOOKEEPER-367):

zookeeper-server/src/test/resources/data/invalidsnap/version-2/snapshot.83f

This status quo creates a number of problems:

1. It makes the test ineffective after the first run;
2. The file shows as modified in version control tools, which can be annoying;
3. The "fixed" snapshot can end up being committed by mistake, invalidating the test.

(#3 is not theoretical; that "fixed" snapshot frequently shows up in pull requests, and was recently merged into master.).