Support a more flexible fs layout than embedding _engine within the test root dir

[xparq]

Related ("dependent") issues (decisions that have simplified things a lot):

• TEST_DIR/run (and run.cmd) has been used as the sole reliable anchor for locating the TEST_DIR.. Where that script is, there's the test stuff. And from there it just also knows that _engine is also there, right underneath.

• The current logic is too brittle: it relies solely on $0 being in the _engine dir, which is not the case when run is being sourced from a script elsewhere, or if it was run via a symlink (at least on Windows; can't recall if sh resolves that symlink, hopefully not)!

• Now at least TEST_DIR can be set before calling run (which would then accept it, no questions asked).

• Support symlinks to _engine/run[.cmd] as anchors of the test dir!
  -> E.g.: https://github.com/cavo789/dos_batch_tips#getsymlinktargetpath