Fix hardcoded use of target/trash in LocalDiskRepositoryTestCase

Fix hardcoded use of target/trash in LocalDiskRepositoryTestCase

`pwd`/target is only valid in Maven Reactor builds where Maven
has moved into the project directory and created a target for
the build output. Most other build systems do not use "target"
and may not even perform a directory change between test suites.

Rewrite LocalDiskRepositoryTestCase's temporary directory code
to use the system specified location and create new unique names.
This prevents fixes between concurrently running tests and allows
the caller to specify the root using java.io.tmpdir.

Update the surefire command lines to use target within each project as
the system temporary directory during unit testing, preventing JGit's
own test suite from writing to /tmp or somewhere like C:\tmp.

Change-Id: I9e8431f6ddfc16fee89f677bcce67c99cfb56782