vg/vgtex: test leaves the git working directory dirty

Created by: kortschak

The vg.Example writes "vg/vgtex/testdata/scatter.tex" which does not exist in the repository, leave the git working directory dirty. This should not happen. The example should follow the approach used by the other examples (see the plotter package). Note that the test also depends on the example test running before TestTexCanvas which is not guaranteed.