Install the test binary and test data. Fixes #241

The resulting install tree, when tests are enabled, looks like this:

```
.
├── cmake
│   ├── nlohmann_jsonConfig.cmake
│   ├── nlohmann_jsonConfigVersion.cmake
│   └── nlohmann_jsonTargets.cmake
├── include
│   └── nlohmann
│       └── json.hpp
└── test
    ├── bin
    │   └── json_unit
    └── data
        ├── json_nlohmann_tests
        │   ├── all_unicode.json
        │   └── bom.json
        ├── json.org
        │   ├── 1.json
        │   ├── ...
        ├── json_roundtrip
        │   ├── roundtrip01.json
        │   ├── roundtrip02.json
        │   └── ...
        ├── json_tests
        │   ├── fail10.json
        │   └── ...
        └── json_testsuite
            └── sample.json
```

It has the property that you can invoke the test binary from the
root of the install tree and the tests work correctly (you no
longer depend on the test binary being run inside the source
tree).

If tests are disabled, the entire `test/` subtree is omitted.
Notice how that yields exactly what you want for using this
library in other projects.

I do not believe I need to update travis due to this change, as the
evil Makefile continues to do in-tree builds. I expect I'll find
out soon enough.

test/CMakeLists.txt

@@ -11,5 +11,11 @@ set_target_properties(json_unit PROPERTIES
     COMPILE_OPTIONS "$<$<CXX_COMPILER_ID:MSVC>:/EHsc;$<$<CONFIG:Release>:/Od>>"
 )
 
+# Install the test binary.
+install(TARGETS json_unit RUNTIME DESTINATION test/bin)
+
+# Copy the test data to the install tree.
+install(DIRECTORY data/ DESTINATION test/data)
+
 target_include_directories(json_unit PRIVATE "src")
 target_link_libraries(json_unit ${JSON_TARGET_NAME})