gtest cmake: fix unit test filter

[MaEtUgR]

Solved Problem

Unit test filtering doesn't work anymore on main since https://github.com/PX4/PX4-Autopilot/pull/23869/files#r1841024972

Solution

Seems that apparently the parameters cannot be assembled in a CMake string variable like that so I'll just explicitly add the -r argument in the separate already existing TESTFILTERARG variable.

Changelog Entry

Fix: Unit test filter

Test coverage

Works again like expected on 22.04. @mrpollo please check if it makes the error you described in https://github.com/PX4/PX4-Autopilot/pull/23869/files#r1841089807 reappear. I couldn't guess where you had it but it should still be fixed.

[github-actions]

FLASH Analysis

px4_fmu-v5x

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  +0.0%      +8  +0.0%      +8    .text
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px4_fmu-v6x

    FILE SIZE        VM SIZE    
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  +0.0%      +8  +0.0%      +8    .text
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