Fix missing group_by default value in string_matching macros

[samantha-guerriero-cko]

Fix for issue #125.

Problem: All the methods in macros/schema_tests/string_matching/ define a group_by_columns=group_by functionality, but group_by is not defined in the method which makes them fail with 'MacroGenerator' object is not iterable.

Solution: we set group_by = None as it's not used by the method.

[clausherther]

@samantha-guerriero-cko thanks for the super quick turnaround here! Will do a release later today, but may take a bit before it shows up on the dbt package hub.

[samantha-guerriero-cko]

thank you @clausherther 💪