fix: undefined context.filename for inline lint

[david-vaclavek]

Fixes #162

[vitonsky]

Currently this code just hides a problem for exists test cases.

Let's add tests that reproduce original problem from issue, and make sure all other tests will be run for that case too.

[david-vaclavek]

@vitonsky
I'm not sure it is possible to properly cover it with the tests.

See the screenshot:

1. Sample updated test.
2. Test console.log statement.
3. console.log output, which proves that both context.filename and context.getFilename() return the same value. If filename in test definition would be undefined, then both values are also undefined. It's not possible to pass getFilename: () => {...} function into test definition, as it's not part of the interface.

My conclusion:
Testing the behavior of context.getFilename() in ESLint can be done using the RuleTester class by passing a filename property in the test cases. It's a part of ESLint 7 usage that context.getFilename() must be used.

Reading the statement above, how exactly would you like me to cover it with tests? Could you provide an example?