Separate tests for preamble, `lib/`, and `rt/`

[ssoelvsten]

With #54 , I've been laying out the foundations for a standard library. While doing so, I only ran the new test file in tests/lib/ rather than the entire test suite. Yet, since some rt/core tests depend on the same library, my pull request broke. But, the library is not part of the core of Troupe and should not be tested there.

The goal of tests is to document intended behaviour and to guide debugging efforts. Since most programs in tests/ are system tests, i.e. are running the entire pipeline rather than a single module, the tests ought to adhere to the criteria below. If this is achieved, then the combination of succeeding and failing tests can more quickly guide the developer towards the origin of a bug.

Test Design Criteria:

1. Tests should only test the one thing as is documented in its name and/or a comment.
2. Tests should be independent of code that is not directly intended to be under test.

Examples of tests violating the above:

• tests/rt/pos/core/float_ops.trp is testing multiple operations [1]. It ought to be split into several files.
• tests/rt/pos/core/float_ops.trp (and many, many other tests) uses print which is part of the preamble and not the Troupe core [2]. Instead, fwrite should be used.
• tests/rt/pos/core/string_manipulations.trp is superseeded by tests/lib/String.trp and should be deleted [2].