Btest

* Port to Python 3. (Daniel Thayer)

* Various cleanup, bug fix, simplifications, and smaller
improvements. (Daniel Thayer)

* Improve and extend test suite substantially. (Daniel Thayer)

* Fix a link in the README. (Jon Siwek)

* Work-around for systems reporting that a socket path is too long.
Addresses BIT-862. (Robin Sommer)

* Fix for Linux systems that have the perf tool but don't support
measuring instructions. (Robin Sommer)

* No longer tracking tests that are expected to fail in state file.
(Robin Sommer)

* Refactoring the timing code to no longer execute at all when not
needed.(Robin Sommer)

* Correct typos in documentation. (Daniel Thayer)

* Failed tests are now only recorded into the state file when we're
not updating. That allows to run "btest -r" repeatedly while
updating baselines in between. (Robin Sommer)

* Experimentation Sphinx directive to write a btest with a Sphinx
document. See README for more information.

* Fixing typos, plus an console output tweak. (Robin Sommer)

* Option -q now implies -b as well. (Robin Sommer)

* New command to copy a file into a test's directory.

``TEST-COPY-FILE: <file>``
Copy the given file into the test's directory before the test is
run. If ``<file>`` is a relative path, it's interpreted relative
to the BTest's base directory. Environment variables in ``<file>``
will be replaced if enclosed in ``$
{
..
}``. This command can be
given multiple times. (Robin Sommer)

* Suppressing error messages when btest-diff can't remove diag file.
(Robin Sommer)

* Adding option -q/--quiet to suppress informational non-error
output. (Robin Sommer)

* Option -F also takes a comma-separated list to specify multiple
filters , rather than having to give -F multiple times. (Robin
Sommer)