test_source: do not instantiate Source objects during collection

testing/code/test_source.py

@@ -127,7 +127,8 @@ def test_isparseable():
 
 
 class TestAccesses:
-    source = Source(
+    def setup_class(self):
+        self.source = Source(
             """\
             def f(x):
                 pass
@@ -155,7 +156,8 @@ class TestAccesses:
 
 
 class TestSourceParsingAndCompiling:
-    source = Source(
+    def setup_class(self):
+        self.source = Source(
             """\
             def f(x):
                 assert (x ==
@@ -619,7 +621,8 @@ x = 3
 
 
 class TestTry:
-    source = """\
+    def setup_class(self):
+        self.source = """\
 try:
     raise ValueError
 except Something:
@@ -646,7 +649,8 @@ else:
 
 
 class TestTryFinally:
-    source = """\
+    def setup_class(self):
+        self.source = """\
 try:
     raise ValueError
 finally:
@@ -663,7 +667,8 @@ finally:
 
 
 class TestIf:
-    source = """\
+    def setup_class(self):
+        self.source = """\
 if 1:
     y = 3
 elif False: