binman: Run tests concurrently

At present the tests run one after the other using a single CPU. This is
not very efficient. Bring in the concurrencytest module and run the tests
concurrently, using one process for each CPU by default. A -P option
allows this to be overridden, which is necessary for code-coverage to
function correctly.

This requires fixing a few tests which are currently not fully
independent.

At some point we might consider doing this across all pytests in U-Boot.
There is a pytest version that supports specifying the number of processes
to use, but it did not work for me.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/py/README.md

@@ -29,6 +29,7 @@ tests. Similar package names should exist in other distributions.
 | -------------- | ----------------------------- |
 | python         | 2.7.5-5ubuntu3                |
 | python-pytest  | 2.5.1-1                       |
+| python-subunit | -                             |
 | gdisk          | 0.8.8-1ubuntu0.1              |
 | dfu-util       | 0.5-1                         |
 | dtc            | 1.4.0+dfsg-1                  |