tests: Allow turning on shell tracing from environment variables

tests: Allow turning on shell tracing from environment variables

This allows turning on shell tracing for grub-shell and grub-fs-tester
when its not practical or not possible to use command line arguments
(e.g. from "make check"). Turn on tracing when the envvar is an integer
greater than 1, since these can generate a lot of output. Since this
change uses the environment variables to set the default value for debug
in grub-shell, this allows enabling grub-shell's debug mode which will
preserve various generated output files that are helpful for debugging
tests.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>