The previous mock library assumed the DRM client would always set all
properties in the atomic request. However, if a previous test-only commit
has set a property, there's no need to set it again.
Accomodate for this in libdrm_mock. This will be useful when we'll skip
setting properties that haven't changed in libliftoff.
References: https://github.com/emersion/libliftoff/issues/37
When testing the library, the libdrm dependency is swapped with a mock libdrm.
The test sets up the mock libdrm's internal state and then runs the test.
