Fix BigIndexTest to work in IDE's w/out sys.prop

Attached is a small patch to fix com.healthmarketscience.jackcess.BigIndexTest to run in an IDE.
The problem was a NullPointerException in tearDown() when no system property for Database.USE_BIG_INDEX_PROPERTY is set.

BigIndexTest.tearDown() changes to :

@Override
protected void tearDown() {
if (_oldBigIndexValue != null) {
System.setProperty(Database.USE_BIG_INDEX_PROPERTY, _oldBigIndexValue);
} else {
System.getProperties().remove(Database.USE_BIG_INDEX_PROPERTY);
}
}

Note: I've already applied this change to the 'newformats' branch.

Dan Rollo