tests: make filterpyflakes.py read target files relatively to cwd

Before this patch, 'filterpyflakes.py' reads target files relatively
to own location.

But this prevents third party tools from using it in own source tree,
because their files are placed separately from 'filterpyflakes.py'.

In fact, 'test-check-pyflakes.t', which is the only user of
'filterpyflakes.py', changes current working directory (cwd) to the
root of "test target" source tree before using it. Therefore,
composing the root of source tree in 'filterpyflakes.py' is redundant.

This patch makes 'filterpyflakes.py' read target files relatively to
cwd by invoking 'open()' without any path composition. This also
removes importing 'os' module, because there is no user of it after
this patch.

This is a one of preparation of issue4677.

tests/filterpyflakes.py

@@ -2,7 +2,7 b''
 
 # Filter output by pyflakes to control which warnings we check
 
-import sys, re, os
+import sys, re
 
 def makekey(typeandline):
     """
@@ -42,7 +42,7 b' for line in sys.stdin:'
     else:
         continue # no pattern matched, next line
     fn = line.split(':', 1)[0]
-    f = open(os.path.join(os.path.dirname(os.path.dirname(__file__)), fn))
+    f = open(fn)
     data = f.read()
     f.close()
     if 'no-' 'check-code' in data: