fixed cmake setup when tests are disabled

[debaetsd]

Description

This PR fixes cmake setup when tests are explicitly disabled ( -DOSL_BUILD_TESTS=0 on cmake cmd line).
The (recently introduced) testing cmake file is only included when that option is enabled yet adding the tests themselves is not checked (and so cmake does not know osl_add_all_tests).

Tests

tested local with/without changes (with these changes it works as expected)

Checklist:

• I have read the contribution guidelines.
• I have previously submitted a Contributor License Agreement.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

LGTM!