llext: tests: re-initialize variables

Some tests depend on global initialized variables, which are then
modified during the test. When enabling user mode, the functions are
called multiple times but the variables are not reinitialized, resulting
in a test failure.

This patch adds a run_id variable to the tests to reinitialize the
variables when the test is called multiple times. It is purposefully
initialized to 41 at startup to detect if the variable is not set up
properly by the llext loader.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>