e2e-tests: Reduce flakiness of Java symbol bar test

Created by: mrnugget

This addresses the recent flakiness of this test by by choosing a smaller Java repository and file with less symbols that all fit into the symbol bar.

The assumption underlying this change is explained in this Slack message:

Before this change, the number of symbols in the given subfolder was so high that only a subset could be displayed (to see the others, one would have to scroll).

The suspicion is that this list of symbols is unordered, which meant that sometimes the desired symbol was visible and sometimes not.

This change here switches from the gitblit/gitblit repo to sourcegraph/java-langserver and changes the subfolder to be one that contains only the 19 symbols found in the single file. In the configured viewport, all of these symbols are visible, independent of sort order.