Remove everything up until `class Foo(object):` in pdb tests

PyPy3 displays prompt two times on the line where `s` is present.

tests/test_shells/pdb.module.ok

@@ -1,5 +1,3 @@
- 1  pdb-script.py:2 <module>  s
--> class Foo(object):
  1  pdb-script.py:6 <module>  
 --Call--
 -> class Foo(object):

tests/test_shells/pdb.subclass.ok

@@ -1,9 +1,3 @@
---Call--
--> def nop(_):
- 2  <string>:1 <module>  pdb-script.py:2 <module>  
--> def nop(_):
- 2  <string>:1 <module>  pdb-script.py:2 <module>  
--> class Foo(object):
  2  <string>:1 <module>  pdb-script.py:6 <module>  
 --Call--
 -> class Foo(object):

tests/test_shells/postproc.py

@@ -38,10 +38,11 @@ with codecs.open(fname, 'r', encoding='utf-8') as R:
 		for line in (R if shell != 'fish' else R.read().split('\n')):
 			i += 1
 			if not found_cd:
-				if shell == 'pdb':
-					found_cd = (i > 1)
-				else:
-					found_cd = ('cd tests/shell/3rd' in line)
+				found_cd = (
+					'class Foo(object):' in line
+					if shell == 'pdb' else
+					'cd tests/shell/3rd' in line
+				)
 				continue
 			if 'true is the last line' in line:
 				break