Check saved_errors before and after every test

Check saved_errors before and after every test

Add a new buptest module that includes a no_lingering_errors context
manager which provokes a wvtest failure if there are any saved_errors
before or after a test, and then clears them, and use that manager in
all of the Python tests so that unexpected errors will be detected
immediately, instead of causing an unrelated failure later, when testing
for expected saved_errors.

Add a temp_testdir context manager to replace all of the current
tmpdir-related boilerplate.

Rework all of the tests to use the two managers where appropriate, so
that this:

  def test_bloom():
      initial_failures = wvfailure_count()
      tmpdir = tempfile.mkdtemp(dir=bup_tmp, prefix='bup-tbloom-')
      ...
      if wvfailure_count() == initial_failures:
          subprocess.call(['rm', '-rf', tmpdir])

becomes this:

  def test_bloom():
      with no_lingering_errors(), test_tempdir('bup-tbloom-') as tmpdir:
          ...

While making the changes, clean up the imports to match the PEP 8 import
recommendations a bit more closely (excepting the suggestion to avoid
"import bar, baz", which seems odd given the blessing of "from foo
import bar, baz", where bar and baz could still be modules).

Thanks to Tadej Janež for reporting the initial issue.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>