Make tests opt-in via `-DSDPTRANSFORM_BUILD_TESTS=ON`

[regexident]

This change makes test compilation opt-in (analog to how libmediasoupclient does it), resulting in a consistent build-time reduction of ~60%.

Build with tests enabled

time cmake . -Bbuild -DSDPTRANSFORM_BUILD_TESTS=ON

Executed in  913.33 millis    fish           external
   usr time  410.18 millis  150.00 micros  410.02 millis
   sys time  391.09 millis  857.00 micros  390.23 millis

time cmake --build build

Executed in   21.44 secs    fish           external
   usr time   20.11 secs  143.00 micros   20.11 secs
   sys time    1.12 secs  970.00 micros    1.12 secs

---

Build with tests disabled

time cmake . -Bbuild -DSDPTRANSFORM_BUILD_TESTS=OFF

Executed in  869.65 millis    fish           external
   usr time  419.19 millis  167.00 micros  419.02 millis
   sys time  378.94 millis  891.00 micros  378.05 millis

time cmake --build build

Executed in    8.85 secs    fish           external
   usr time    8.21 secs    0.20 millis    8.21 secs
   sys time    0.56 secs    1.52 millis    0.56 secs