twister: Include unit test `testbinary` in ELF search space

`TestInstance.get_elf_file()` does not include the binary name used for
unit testing (`testbinary`) in its search, causing unit tests to not
complete successfully. This does not cause tests to fail and thus goes
unnoticed.

Update the glob expressions used in `get_elf_file()` to also look for
files called `testbinary`

Signed-off-by: Tristan Honscheid <honscheid@google.com>