test: py: test_shell_run() with CONFIG_HUSH_PARSER=n

The hush parser not enabled for some boards, e.g.
sipeed_maix_bitm_defconfig.

With CONFIG_HUSH_PARSER=n a double quotation mark is not interpreted as the
beginning of a string. Use a single quotation mark instead.

Furthermore without the hush parser variables have to be referenced as
${varname}. Add the missing braces.

Reported-by: Sean Anderson <seanga2@gmail.com>
Fixes: 8b86c609 ("test/py: add test of basic shell functionality")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>