test/py: strip VT100 codes from match buffer

test/py: strip VT100 codes from match buffer

Prior to this patch, any VT100 codes emitted by U-Boot are considered part
of a command's output, which often causes tests to fail. For example,
test_env_echo_exists executes printenv, and then considers any text on a
line before an = sign as a valid U-Boot environment variable name. This
includes any VT100 codes emitted. When the test later attempts to use that
variable, the name would be invalid since it includes the VT100 codes.
Solve this by stripping VT100 codes from the match buffer, so they are
never seen by higher level test code.

The codes are still logged unmodified, so that users can expect U-Boot's
exact output without interference. This does clutter the log file a bit.
However, it allows users to see exactly what U-Boot emitted rather than a
modified version, which hopefully is better for debugging. It's also much
simpler to implement, since logging happens as soon as text is received,
and so stripping the VT100 codes from the log would require handling
reception and stripping of partial VT100 codes.

Signed-off-by: Stephen Warren <swarren@nvidia.com>