test_ocr_on_live_video: Fix timeout when run under load

When run under heavy load (e.g. `make check` with GNU parallel
installed) this self-test could fail if the video pipeline hadn't
managed to start at all in the 10 seconds allowed by the call to
`get_frame` inside of `stbt.ocr()`.
Selftest that `run-tests.sh` isn't affected by user's $STBT_CONFIG_FILE

This test currently fails, but will be fixed by the next commit.
tests/run-tests.sh [fix]: unset STBT_CONFIG_FILE so no user override

Commit c83c42a2 "improved sandboxing" by creating an isolated environment
to run tests in. This included making a copy of test/stbt.conf and
putting into a pseudo system-default config directory. However, if the
user already has a STBT_CONFIG_FILE environment variable set, this will
take precendence over the new "system config".

This commit adds "unset STBT_CONFIG_FILE". I tested that unsetting an
environment variable does not carry through to the executing shell once
run-tests.sh has ended, so the user doesn't have to re-set it.
stbt-templatematch: use sensible, canonical variable names

This commit attempts to make the variable names used for the two image
file inputs and their corresponding numpy arrays, which previously were
mixed up and confusing.

For example, the file name of the template was stored in the variable
`template_image`, and then the image itself stored in `template`. Yet
`image` is the variable containing the `full_frame` numpy array, and was
given as the value to MatchResult.frame, and the not-yet used but optional
parameter MatchResult.image would (theoretically) be `template_image`.
Most confusing is the call to `stbt._match`, which had `image`, `template`
and `template_image` as arguments!

To make this much less confusing, I've adopted the naming convention used
by stbt templatematch's verbose debug output, where the input frame from
the video source is called `source` and the `template` is called `template`
and further clarified them with `_image` and `_file` suffixes.
stbt-templatematch: create MatchResult with `image` to avoid warning

Commit f51f6487 introduced the `image` parameter to the MatchResult class,
and added a DeprecationWarning which was invoked whenever a MatchResult
was created without `image` being set. However, the `image` parameter
wasn't added to stbt-templatematch, and so the verbose DeprecationWarning
came up every time you ran it.

Note that I could have added `image=template_image` and it would have
been valid, however this would print out "image=<Custom Image>", which
is pretty useless.
Merge branch 'stbt-templatematch--no-depr-warning'

* stbt-templatematch--no-depr-warning:
  stbt-templatematch: create MatchResult with `image` to avoid warning
  stbt-templatematch: use sensible, canonical variable names
  tests/run-tests.sh [fix]: unset STBT_CONFIG_FILE so no user override
  Selftest that `run-tests.sh` isn't affected by user's $STBT_CONFIG_FILE

Closes #142.