All tests run / only previous exercise runs

[zwilias]

On current master (c4b2440e7e883c0b46e5d27d9ac64627ea951ec4), attempting to run the tests for a single exercise gives some issues.

To reproduce:

• clone the repo
• run npm i
• run npx elm-test
• run npx elm-test tests/Exercise01

The second invocation is expected to execute only the tests for the first exercise, but instead runs them all.

Alternative scenario:

• clone the repo
• run npm i
• run npx elm-test tests/Exercise01
• run npx elm-test tests/Exercise02

The second invocation is expected to run the tests for the second exercise, but instead runs the tests for the first exercise again.

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Current workaround: delete elm-stuff/generated-code when switching between exercises.