make check-pylint: Fix false negatives introduced in 72219bdb

Surprisingly (to me, at least) the change in 72219bdb causes `$r`
to *not* be set to `1` when `pylint` fails. The following experiment
confirms the confusing behaviour:

    $ r=0; { echo hi; false || r=1; }; echo $r
    hi
    1

    $ r=0; { echo hi; false || r=1; } | grep -v hi; echo $r
    0
stbt.py: Fix pylint errors introduced in b95dc8e7

These errors were not picked up by `make check` because of the bug fixed
in 72ab49b6.

The errors were:

    stbt.py:794: Attribute 'start_timestamp' defined outside __init__
    stbt.py:877: Attribute 'source_pipeline' defined outside __init__
stbt run: Consistent capitalisation of help message

The placeholder names for varying text provided by the user ("CONTROL",
etc.) are all uppercase; "SCRIPT" is now consistent with the other
names.
stbt run: `script` positional argument cannot be specified in config file

It never made sense for the script name to default to a value fetched
from stbt.conf, because unlike the source video pipeline and the remote
control mechanism, you'll probably want to run a different test script
each time you invoke `stbt run`.

This change is necessary for the next commit, so that the script name
is not ambiguous with arguments passed to the script.
stbt run: Pass extra command-line arguments to test script

This will allow a more tabular approach when you have many permutations
to test. For example, you could write a shell script like this:

    stbt run my-test.py 0 0
    stbt run my-test.py 0 1
    stbt run my-test.py 1 0
    stbt run my-test.py 1 1

where `my-test.py` is a script that takes 2 command-line arguments
controlling particular test parameters.

This is often neater than passing such information in environment
variables or in stbt.conf.