Review naming and packaging in `...testutil...` modules and corresponding published artifacts

[armiol]

Right now there is a discrepancy between testutil modules in core-java and those is time, web, and (now) logging.

Their root package names are different by their composition idea. Some of them are io.spine.testing.<foobar>, while others are io.spine.<foobar>.testing.

Also, all of the artifacts names are built by <group-ID>:spine-testutil-<module-name> pattern. It is somewhat weird given that there is no testutil nowhere near in the package name.

A good time to revise these things is 2.0 release.

To do in scope of this issue:

• Decide on consistent naming of packages across testutil modules.
• Decide whether we want to name the published artifact somewhat differently. E.g.
  • Switch to testing instead of testutil (and rename modules as well).
  • Swap the module name and testutil: have <group-ID>:spine-<module-name>-testing or <group-ID>:spine-<module-name>-testutil.