Currently, when you build on an M1 Mac you have to manually specify a gn argument to build for an arm64 CPU. Because of the way that the unit tests are enabled, this will turn off the unit test builds by default unless you manually specify the --enable-unittests command line argument to gn. This problem likely is due to the following line of code:

https://github.com/flutter/engine/blob/d366183c03c9e12b8e0845ed2f35c6c091a29627/testing/testing.gni#L15