[Services] Use BUILD_TESTING for tunnelbroker build

Summary:
The biggest change is building the testsuite by default, however, this is
the common practice, and generally people opt-out of building tests by doing
-DBUILD_TESTING=OFF when they invoke cmake.

https://linear.app/comm/issue/ENG-1794

Test Plan:

nix develop
cd services/tunnelbroker
rm -rf build && mkdir build && cd build && cmake .. && make -j
ctest # should fail as AWS needs to be configured, but run

# without tests
nix develop
cd services/tunnelbroker
rm -rf build && mkdir build && cd build && cmake .. -DBUILD_TESTING=OFF && make -j
ctest # should fail to find tests

Reviewers: atul, abosh, varun, max, karol

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5118