Migrate test setup/teardown to fixtures

[zbeekman]

Right now, the tests are somewhat stateful: Tests modify inputs/outputs of other tests and/or don't clean up after themselves properly. This can cause problems when JSON-Fortran is added as an ExternalProject in a CMake "SuperBuild" with testing enabled.

What happens is that subsequent runs of the build will see that JSON-Fortran is up to date, skipping the configure and build steps, but will run the test step. (Or, perhaps it will re-run the configure step I'm not 100% clear on this, but I think my initial understanding is correct.) Because the test inputs/outputs are stateful, the initial run after configuring and building will pass, but subsequent runs will fail.

To resolve this the following steps should be taken:

1. Analyze inter-test run order dependencies. CTest allows tests to be run in parallel, and some tests may be taking the output of others as inputs. This should be explicitly declared.
2. CMake/CTest has added support for "test fixtures" which allow setup/teardown tasks for groups of tests to be declared. Use this to copy inputs (and cleanup outputs) between test runs.