[!124] [cmake][bugfix] Allow test executables to be located elsewhere

Dominic Kempfrequested to merge
cherry-pick-5aa6e1a1 into releases/2.6

Merge branch 'bugfix/test-executable-from-other-directories' into 'master'

See merge request quality/dune-testtools!124

(cherry picked from commit 5aa6e1a1)

ec756543 [cmake][bugfix] Allow test executables to be located elsewhere

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