Automatic result path for assertString().

As another step towards #884 (and a follow-up to #1471) this PR:
 1. Puts `assertString` on a par with `assertCML` and `assertCDL` in providing a sensible default result path (derived from the module, class, and method name).
 2. Updates one of the test classes to use the new defaults, as a demonstration.
 3. Gets rid of an unused parameter in `_check_same()`.
  - NB. This also required updating `iris.tests.experimental.test_raster` to avoid using this method.

~~NB. If you're feeling paranoid it might be worth waiting for #1473 to be merged and rebasing to ensure all the result files line up.~~ #1473 has already been merged and this PR rebased.