Fix running Compiler tests standalone (#56952)

Author: Keno

Compiler/src/bootstrap.jl

@@ -15,6 +15,7 @@ function activate_codegen!()
     end)
 end
 
+global bootstrapping_compiler::Bool = false
 function bootstrap!()
     global bootstrapping_compiler = true
     let time() = ccall(:jl_clock_now, Float64, ())