twister: Switch to a non-greedy regex

When parsing test names from symbol names, use a non-greedy match to
avoid parsing test names with __ in them.

The symbol looks like z_ztest_unit_test__SUITE__TEST. It is more likely
that a test name will contain __ than a suite name, so now something
z_ztest_unit_test__a_b__c_d__e_f will be parsed as suite:a_b
test:c_d__e_f, whereas it used to be parsed as suite:a_b__c_d test:e_f

Signed-off-by: Jeremy Bettis <jbettis@google.com>