Add more integration tests

[loreanvictor]

Some edge cases are particularly difficult to pinpoint in unit tests (for example #15, #14 ). Some integration tests on the tmplr repository itself would help mitigate such regressions.

An incremental strategy for this would be recommended: instead of trying to cover everything, it might be more productive to add tests case by case, trying to have each case cover a range of cases.

[loreanvictor]

The setup for e2e testing, alongside some basic scenarios (including the regression mentioned above), and utilities for easier e2e testing, are added. I'll keep this issue open to add more tests over time.