tests/test-filecache.py.out

testing: allow Hypothesis to enable extensions

This adds support for testing extensions, including both tests
that extensions don't change behaviour and test for specific
commands.

We use the precondition system to determine what commands are
available to us. If we never use any commands enabled by an
extension then that extension is *skippable* and should not
have changed the behaviour of the test. We thus rerun the test
with an environment variable which is designed to turn off the
extension.

basic:

* neither file exists
creating
* neither file still exists
* empty file x created
creating
* file x changed size
creating
* nothing changed with either file
* file x changed inode
creating
* empty file y created
creating
* file y changed size
creating
* file y changed inode
creating
* both files changed inode
creating

fakeuncacheable:

* neither file exists
creating
* neither file still exists
creating
* empty file x created
creating
* file x changed size
creating
* nothing changed with either file
creating
* file x changed inode
creating
* empty file y created
creating
* file y changed size
creating
* file y changed inode
creating
* both files changed inode
creating
repository tip rolled back to revision -1 (undo commit)
working directory now based on revision -1
repository tip rolled back to revision -1 (undo commit)
working directory now based on revision -1

setbeforeget:

* neither file exists
string set externally
* file x created
creating
string from function
* string set externally again
string 2 set externally
* file y created
creating
string from function