MiniMaven: avoid depending on ij-core:tests

MiniMaven's tests used ij-core's tests' createTemporaryDirectory() method.
That comprised all of ij-core's functionality MiniMaven used.

Let's just copy that function so that the path is open to move MiniMaven
into its own repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: replace ij-core dependency with scijava-common

MiniMaven only relies on classes contained in scijava-common, so let's
not pretend that it relies on ij-core.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make MiniMaven a stand-alone project

While at it, rename the artifact and change the version to
1.0.0-SNAPSHOT to indicate that we are very close to releasing a first
official MiniMaven release.

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