When converting test/py from " to ', I missed a few places (or added a
few inconsistencies later). Fix these.

Note that only quotes in code are converted; double-quotes in comments
and HTML are left as-is, since English and HTML use " not '.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Python's coding style docs indicate to use " not ' for docstrings.

test/py has other violations of the coding style docs, since the docs
specify a stranger style than I would expect, but nobody has complained
about those yet:-)

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

This tests whether the following features of the U-Boot shell:
- Execution of a directly entered command.
- Compound commands (; delimiter).
- Quoting of arguments containing spaces.
- Executing commands from environment variables.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>