add regex validator for project test name

[Syuparn]

This PR adds a test name validator to prevent project crashes.

Since test names cannot have special characters, creating a new test with a special character name causes an error.
Furthermore, once a test with an invalid name has been created in a project, every page in the project throws the same error and the project cannot be used anymore.

(before: creating a new test with a special character name)

(before: top page in the project throws an error)

NoReverseMatch at /user1/foobar/
Reverse for 'test' with arguments '(<User: user1>, 'foobar', 'test!')' not found. 1 pattern(s) tried: ['(?P<user>[^/]+)/(?P<rule>[-a-zA-Z0-9_]+)/test/(?P<test_name>[-a-zA-Z0-9_]+)/\\Z']

This PR adds a validator to the BlawxTest model to validate its name before saving it so that any other pages in the project can be accessed afterwards.

(after: a new test with a special character name is validated before saved)

(after: top page in the project can be accessed)