updated make help message

#### Summary:

Updates the make help message for CmdStan.

#### Intended Effect:

When `make help` is executed, the message explaining the test targets are clearer.

#### How to Verify:

From the command line, type:
`make help`

The documentation for tests is clearer than it was before. This does not affect any of the tests.

#### Side Effects:

None.

#### Documentation:

None.

#### Reviewer Suggestions:

Anyone. Bob suggested the updated language.