build: TestSetup.exe_wrapper doesn't need to be optional

It works fine as-is with an empty list, and since that's easier to get
using our typed_kwargs, and thus is what we're passing, go ahead and
simplify the class to only take a list of strings.

mesonbuild/build.py

@@ -2796,7 +2796,7 @@ class Data(HoldableObject):
         self.data_type = data_type
 
 class TestSetup:
-    def __init__(self, exe_wrapper: T.Optional[T.List[str]], gdb: bool,
+    def __init__(self, exe_wrapper: T.List[str], gdb: bool,
                  timeout_multiplier: int, env: EnvironmentVariables,
                  exclude_suites: T.List[str]):
         self.exe_wrapper = exe_wrapper