[TODO] Change JSON config language API

[JustinMeimar]

The current API is coupled to a single runtime library config. It can be extended to a generic ENV field for each executable.
With this we can solve platform compatibility between macOS and linux at the config level and save several LOC that handle shared library setup internally. Making ENV variables first class also just makes the tester more generic.

The reason this was not done in F24 is because it is an API breaking change which would require updating all the student configs.

Current config for runtime compilers:

{
  "testDir": "../packages/RuntimeCPackage",
  "testedExecutablePaths": {
    "gcc": "/usr/bin/clang"
  },
  "runtimes": {
    "gcc": "../lib/libfib.so"
  },
  "toolchains": {
    "clang-runtime": [
      {
        "stepName": "clang",
        "executablePath": "$EXE",
        "arguments": ["-c", "$INPUT", "-o", "$OUTPUT"],
        "output": "/tmp/prog.o"
      },
      {
        "stepName": "compile",
        "executablePath": "$EXE",
        "arguments": ["$INPUT", "-o", "$OUTPUT", "-L$RT_PATH", "-l$RT_LIB"],
        "output": "/tmp/prog"
      },
      {
        "stepName": "run",
        "executablePath": "$INPUT",
        "arguments": [],
        "usesInStr": true,
        "allowError": true,
        "usesRuntime": true
      }
    ]
  }
}

Proposed Change:
Create a distinction between automatic variables and environment variables (@ and $ respectively)

{
  "testDir": "../packages/RuntimeCPackage",
  "testedExecutablePaths": {
    "gcc": {
       "path":
       "env": {
          ''LD_LIBRARY_PATH": "../lib"
       }
     }
  },
  "toolchains": {
    "clang-runtime": [
      {
        "stepName": "clang",
        "executablePath": "@EXE",
        "arguments": ["-c", "@INPUT", "-o", "@OUTPUT"],
        "output": "/tmp/prog.o"
      },
      {
        "stepName": "compile",
        "executablePath": "@EXE",
        "arguments": ["@INPUT", "-o", "@OUTPUT", "-L$RT_PATH", "-l$RT_LIB"],
        "output": "/tmp/prog"
      },
      {
        "stepName": "run",
        "executablePath": "@INPUT",
        "arguments": [],
        "usesInStr": true,
        "allowError": true,
        "usesRuntime": true
      }
    ]
  }
}