test/py: serial# cannot be overwritten on some devices

On some devices the environment variable serial# cannot be overwritten.
Set the variable only if it is not set.

For our unit test it is sufficient to test if any value for serial-number
is set.

Fixes: 8a5cdf60 ("test: efi_selftest: Do not force serial# setting")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>