Replace spaces with tabs in test/CMakeLists.txt

Summary:
Somehow spaces (0x20) snuck into test/CMakeLists.txt, which confuses Phabricator's renderer. Using tabs (0x09) fixes this. See here for more details.

Depends on D13330.

Test Plan: Phabricator should render the changed code using 2 wide indents.

Reviewers: Fabien, #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13331