Debian package: Build dependencies for selftests

python-yaml is used by tests/validate-ocr.py -- even though `make check`
doesn't run that script, it does run pylint over it.

The tesseract German language pack is needed for
test_ocr.test_user_dictionary_with_non_english_language and
test_ocr.test_that_ocr_reads_unicode.
stbt-control: Remove unnecessary runtime dependency on nose

The functionality in stbt-control doesn't require nose, but it's tests
did.  Unfortunately as we were using `nose.tools.raises` to decorate
a top level function so we had to import `nose.tools.raises` at the top
level intoducing an unnecessary dependency even when not running the
tests.

This is a little less declarative but is still quite readable and seems to
be better than the alternatives such as moving the test into a different
file or conditionally defining the test function.
Debian package: Install tesseract English language data

Because `stbt.ocr`'s `lang` parameter defaults to English.

On Fedora the English language data is already included in the base
tesseract package.