Make Player tests independent

from GoogleTest documentation:
For each test defined with TEST_F(), Google Test will:
1. Create a fresh test fixture at runtime
2. Immediately initialize it via SetUp() ,
3. Run the test
4. Clean up by calling TearDown()
5. Delete the test fixture.
Note that different tests in the same test case have different test fixture objects,
and Google Test always deletes a test fixture before it creates the next one.
Google Test does not reuse the same test fixture for multiple tests.
Any changes one test makes to the fixture do not affect other tests.
Remove redundant, commented-out code
Move constants from JsonTest and JsonParserTest to header file