tests: cleanup some shellcheck warns

tests: cleanup some shellcheck warns

Reduce shellcheck warnings about missing {} for possible array
dereference.

Make sure we are not loosing error code when assigning local vars
and explicitely ignore 'errors' from standalone lines when needed.

Add some missing quotes.

Use $() instead of ancient ``

Avoid writing some temporary data into /tmp - test need to store
files within its own 'testdir' - so it can be properly discarded.