test: use #nmigen: magic comment instead of monkey patch.

Also, fix missing and incorrect src_loc_at arguments where
appropriate so the testsuite passes without warnings.

nmigen/test/__init__.py

@@ -1,6 +0,0 @@
-from ..hdl.ir import Elaboratable
-
-
-# The nMigen testsuite creates a lot of elaboratables that are intentionally unused.
-# Disable the unused elaboratable check, as in our case it provides nothing but noise.
-del Elaboratable.__del__

nmigen/test/compat/test_coding.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 import unittest
 
 from ...compat import *

nmigen/test/test_build_res.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from .. import *
 from ..hdl.rec import *
 from ..lib.io import *

nmigen/test/test_hdl_dsl.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from collections import OrderedDict
 from enum import Enum
 

nmigen/test/test_hdl_ir.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from collections import OrderedDict
 
 from ..hdl.ast import *

nmigen/test/test_hdl_mem.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from ..hdl.ast import *
 from ..hdl.mem import *
 from .utils import *

nmigen/test/test_hdl_xfrm.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from ..hdl.ast import *
 from ..hdl.cd import *
 from ..hdl.ir import *

nmigen/test/test_lib_cdc.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from .utils import *
 from ..hdl import *
 from ..back.pysim import *

nmigen/test/test_lib_fifo.py

@@ -1,3 +1,5 @@
+# nmigen: UnusedElaboratable=no
+
 from .utils import *
 from ..hdl import *
 from ..asserts import *