tests/utils/utils.sh: remove redundancy when setting def. var. values

tests/utils/utils.sh: remove redundancy when setting def. var. values

This patch removes more redundancy in `utils.sh` by using a function
which sets a variable by name if it's empty and exports it.

This makes the file less error-prone: the variable name is written just
once, making it safer to add more.

The

    export "${varname?}"

form is to make ShellCheck happy, even if a simple `export $varname`
would perfectly work in this context.

The function is unset when it's not needed anymore to avoid polluting
the scope of whatever sources `utils.sh`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6e25aa557f4d09f8129121b76b8b3a860ba3ed29