Certain existing e2e tests are not adaptable to external binary testing.
Although it's better for tests to be able to execute against both an
in-process geth or an extra-process arbitrary ethereum client, the
current state of the tests does not allow this in all cases.

This change allows for external binaries to specify a set of test names
to be skipped, including a skip message for why they are skipped.  The
set is specified as a JSON map in a new optional file `test_parms.json`
which can be created in the external adapter folder.

Prior to this change, the external process was indicated only by a path
to a `shim` binary.  This change now instead asks that the external test
be specified by directory, wherein a `shim` binary must exist, and this
new file `test_params.json` may exist.  Other approaches are of course
possible, such as allowing the JSON file to be specified via a command
flag, but it seemed better to simply establish a convention for expected
paths for these external tests to satisfy.

There actually is a single test which already has a `t.Skip()` based on
the presence of the external L2 flag.  I'd though these were all
removed in the last PR, but this one actually is required for an
external op-geth.  So, this skip has been converted to the new method
and acts as a nice test that it is working.