Is purely a convenience, beginning with NUnit 2.5, if a class satisfies the conditions to be a test fixture and specifies at least a method marked with Test, TestCase or TestCaseSource than that class is treated as a test fixture.
The TestFixture attribute is required however for parameterized or generic test fixture because in that case you also specify additional information through the attribute (parameters/concrete types).
More information about the specific requirements is available at TextFixtureAttribute (NUnit 3 Documentation) and
TestFixtureAttribute (NUnit 2.0 / 2.5)