twister: pytest: escape expected command regex

When sending commands containing something like "AT+CFUN" the expected
regex was not escaped and as a result such regex never matched what has
been echoed over serial.

Escape all commands with re.escape() so that echoed shell commands are
always matched, even when they contain special characters (like '+' in the
example above).

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>