Define CMake/CTest tests

[robertmrk]

In #241 @ChrisKitching described that tests on out of source builds fail because the test executable can't find the JSON files it needs. It's great that he noticed this and provided a fix for it, I totally missed this issue when I rewrote the CMake lists file.
In #242 the issue is fixed by installing the test executable and test files. There are two problems with this approach:

1. The test executable is used only once by the users of the library. It's executed only after a successful build and never used again, so there is no need to install it on the system level.
2. For executing the tests in out of source builds we don't need to copy any files at all. In CMake we can define tests with the source directory as their working directory, so tests will be able to find the JSON files no matter where the project is built.

In this PR I defined two CMake/CTest test configurations.

• json_unit_default for running the default tests by executing
  json_unit without any arguments
• json_unit_all for running all the tests by executing json_unit
  with the "*" argument

So the tests can be now executed by building the test target:

make test

or by running ctest in the build directory.

The tests can be selectively executed as well with ctest:

ctest -R json_unit_default

or

ctest -R json_unit_all

[coveralls]

Coverage remained the same at 99.954% when pulling 639d632 on robertmrk:feature/cmake-cleanup into 0a393b8 on nlohmann:develop.

[nlohmann]

Thanks a lot! I am lacking Cmake experience, so I am happy about any contributions.

[robertmrk]

You're welcome. I'm glad I could help.