Test isolation: some test is switching to readonly mode and not switching back on teardown

Here is a triplet of tests that provoke the get-stuck-in-read-only-mode
problem when run together:

lp.services.database.tests.test_readonly.Test_is_read_only.test_cached_value_takes_precedence
lp.services.webapp.tests.test_dbpolicy.LayerDatabasePolicyTestCase.test_WebServiceRequest_uses_LaunchpadDatabasePolicy
lp.services.webapp.tests.test_dbpolicy.LayerDatabasePolicyTestCase.test_WebServiceRequest_uses_ReadOnlyDatabasePolicy

(You can put the above three lines in a file and run the tests with
bin/test --load-list the_file.)

The first test appears to pass, the second then reports

    AssertionError: newInteraction called while another interaction is active.

The third then fails thusly:

Failure in test lp.services.webapp.tests.test_dbpolicy.LayerDatabasePolicyTestCase.test_WebServiceRequest_uses_ReadOnlyDatabasePolicy
Traceback (most recent call last):
_StringException: Traceback (most recent call last):
  File "/home/benji/launchpad/lp-branches/test-failures-1/lib/lp/services/webapp/tests/test_dbpolicy.py", line 237, in test_WebServiceRequest_uses_ReadOnlyDatabasePolicy
    touch_read_only_file()
  File "/home/benji/launchpad/lp-branches/test-failures-1/lib/lp/services/database/tests/readonly.py", line 39, in touch_read_only_file
    assert is_read_only(), "Switching to read-only failed."
AssertionError: Switching to read-only failed.

As a bonus, replace the first test with

lp.services.database.tests.test_readonly.Test_is_read_only.test_caching_in_request

And the same things will happen, except that the third test will switch
to failing this way:

Failure in test lp.services.webapp.tests.test_dbpolicy.LayerDatabasePolicyTestCase.test_WebServiceRequest_uses_ReadOnlyDatabasePolicy
Traceback (most recent call last):
_StringException: Traceback (most recent call last):
  File "/home/benji/launchpad/lp-branches/test-failures-1/lib/lp/services/webapp/tests/test_dbpolicy.py", line 247, in test_WebServiceRequest_uses_ReadOnlyDatabasePolicy
    remove_read_only_file()
  File "/home/benji/launchpad/lp-branches/test-failures-1/lib/lp/services/database/tests/readonly.py", line 55, in remove_read_only_file
    assert not is_read_only(), "Switching to read-write failed."
AssertionError: Switching to read-write failed.