create_signature=always causes test suite failure

If one has create_signature = always (maybe also gpg_signing_key) set in the [DEFAULT] section of ~/.bazaar/bazaar.conf and runs the Bazaar test suite, it fails as follows:

======================================================================
FAIL: bzrlib.branchbuilder.BranchBuilder
----------------------------------------------------------------------
Traceback (most recent call last):
testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/usr/lib/python2.7/doctest.py", line 2226, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for bzrlib.branchbuilder.BranchBuilder
  File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/branchbuilder.py", line 30, in BranchBuilder

----------------------------------------------------------------------
File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/branchbuilder.py", line 45, in bzrlib.branchbuilder.BranchBuilder
Failed example:
    builder.build_snapshot('rev-id', None, [
        ('add', ('', 'root-id', 'directory', '')),
        ('add', ('filename', 'f-id', 'file', 'content\n'))])
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest bzrlib.branchbuilder.BranchBuilder[3]>", line 3, in <module>
        ('add', ('filename', 'f-id', 'file', 'content\n'))])
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/branchbuilder.py", line 259, in build_snapshot
        message_callback=message_callback)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/branchbuilder.py", line 127, in _do_commit
        **kwargs)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/decorators.py", line 218, in write_locked
        result = unbound(self, *args, **kwargs)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/mutabletree.py", line 211, in commit
        *args, **kwargs)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/commit.py", line 290, in commit
        lossy=lossy)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/cleanup.py", line 132, in run
        self.cleanups, self.func, self, *args, **kwargs)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/cleanup.py", line 166, in _do_with_cleanups
        result = func(*args, **kwargs)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/commit.py", line 447, in _commit
        self.rev_id = self.builder.commit(self.message)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/vf_repository.py", line 208, in commit
        self._new_revision_id)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/decorators.py", line 218, in write_locked
        result = unbound(self, *args, **kwargs)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/repository.py", line 909, in store_revision_signature
        signature = gpg_strategy.sign(plaintext)
      File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/gpg.py", line 237, in sign
        raise errors.SigningFailed(self._command_line())
    SigningFailed: Failed to GPG sign data with command "[u'gpg', '--clearsign', '-u', u'7D86500B']"

Temporarily moving ~/.bazaar aside works around this.

The test suite should be properly isolated from whatever happens to be in the home directory of the user running it.