Fixing problems in the database unit testing environment with Acorn

* Removed static map from org.simantics.acorn.FileIO.
  Moved the map into a session service as org.simantics.acorn.FileCache
* Changed org.simantics.db.testing.common.Tests utilities to use Acorn
  instead of ProCore (AcornTestHandler). Did not introduce abstraction
  for this for now.

refs #7855

Conflicts:
	bundles/org.simantics.acorn/src/org/simantics/acorn/FileIO.java

Change-Id: Iec42395cc71b30a4c376a44778b358e3807e6ec9
(cherry picked from commit 03fac449ec81e88669169d671bb386eb93ac060d)
(cherry picked from commit e5c006a3e29dcb1f29ae5bcc21ac28573bd37648)