sg-run-job: testid generation: Process ts more like rest of args

sg-run-job: testid generation: Process ts more like rest of args

Remove ts as a separate parameter to spawn-ts.  The test script now
becomes the first entry in args.

We process it through the arg loop as before.  Currently there are no
calls where the first arg is `+' so the test script name ends up in
both real_args and testid_args.

We split it out of real_args into the ts variable with lshift.
We split it out of testid_args into the deftestid with lshift.

So afterwards in spawn-ts, all the variables (including real_args, ts,
deftestid and testid_args and hence host_testid_suffix) have the
values they would have had before.

Therefore there is no functional change for any existing calls.

However, because the first argument is not treated specially for the
`+' procesing loop, it is now possible to specify `+' as the first
entry in args to spawn-ts (ie where ts used to be) to arrange that the
deftestid (and hence, probably, the testid) is computed using later
arguments.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>