Simplify nested_externs rules in test/CMakeLists.txt
Simplify nested_externs rules in test/CMakeLists.txt


Former-commit-id: aa983b2c1e8ac7f7bbc14803e1bceffa7ee038a2
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide