Increased test coverage on no-mixed-requires

Added test cases for branches that were not being reached and removed an unnecessary if statement.
Increased test coverage for no-shadow

Removed an unreachable block of code, which increased test coverage. Because `checkForShadows` is called only by `FunctionDeclaration` and "FunctionExpression` and those will both always have an upper scope (`Program`, at the top level), it's unnecessary to check to `node.upper`.
Increased test coverage for max-depth

Added a test case that used the default depth threshold to increase test coverage.
Merge pull request #265 from iancmyers/test-increase-coverage

Increase rules test coverage