LegacyOpenerTest: make sure that context is always disposed of

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Work around the LegacyOpenerTest problem reported by Jenkins

The test was failing in headless mode essentially because two different
contexts reuse the same ImageJ "instance" and the image opened in one
test bleeds into the other.

For now, just close() the images opened in 'batch mode'. We will address
this in https://github.com/imagej/imagej-legacy/issues/72.

Thanks, Jory Jenkins!

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'fix-legacy-opener-test'

This fixes a problem with LegacyOpenerTest reported by Jenkins.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>