Add tests for currently untested code

[renefritze]

Moved from https://github.com/wwu-numerik/dune-stuff/issues/69

Non-exhaustive list of code that should have at least rudimentary test coverage. As in does it compile in the usual use-case.

Common

• [ ] ConvergenceStudy, LocalizationStudy (probably used in another module's tests?)
• [ ] filesystem.hh: esp. error conditions for stream creation
• [ ] misc.hh (wraparound_array)
• [ ] random.hh

[renefritze]

• [ ] DUNE_THROW_IF

[renefritze]

See also https://codecov.io/gh/dune-community/dune-xt-common/tree/master/dune/xt/common

[ftalbrecht]

Wow, this looks like an actually useful report!

[tobiasleibner]

Moved to dune-xt.