Re-enable unit tests on CI

[dector]

After merging #470

./bin/unit_tests.sh

will give

FAILURE: Build failed with an exception.

* Where:
Build file '/home/user/projects/kotlin-v3/exercises/build.gradle' line: 12

* What went wrong:
A problem occurred configuring project ':accumulate'.
> Could not find method sourceSets() for arguments [build_6pb57tnj9mm08ok60t9mujsv6$_run_closure1$_closure2$_closure4@345c9cf1] on project ':accumulate' of type org.gradle.api.Project.

BUILD FAILED in 668ms

Gradle isn't configuring subprojects as Kotlin/Java projects somewhy.

Re-enable unit tests for exercises.