Currently each platform's unit tests run a bunch of pigeon commands to generate the files needed for the native unit tests. As discussed in #115168, this is quite slow. The suggestion there of consolidating the files would help, but I would expect that we could also substantially improve the time required to run tests across all platforms by generating all platforms' output for each input file in a single call, rather than one call per platform.

E.g., instead of Windows tests running with just cppHeaderOut and cppSourceOut, and iOS Swift running with just swiftOut, and Android Kotlin running with just kotlinOut, etc. we would run with all of those arguments at once before starting any of the test runs.

(This would require some restructuring in the script, so should be done after #85068)