add some more unreachable code tests

tests/errors/w_unreachable_cf.pyx

+# mode: error
+# tag: werror, unreachable, control-flow
+
+def try_finally():
+    try:
+        return
+    finally:
+        return
+    print 'oops'
+
+def try_return():
+    try:
+        return
+    except:
+        return
+    print 'oops'
+
+def for_return(a):
+    for i in a:
+        return
+    else:
+        return
+    print 'oops'
+
+def while_return(a):
+    while a:
+        return
+    else:
+        return
+    print 'oops'
+
+def forfrom_return(a):
+    for i from 0 <= i <= a:
+        return
+    else:
+        return
+    print 'oops'
+
+_ERRORS = """
+9:4: Unreachable code
+16:4: Unreachable code
+23:4: Unreachable code
+30:4: Unreachable code
+37:4: Unreachable code
+"""