MNT: Fix CI warnings

ResourceWarning: unclosed file <_io.BufferedReader name='/home/runner/work/nibabel/nibabel/virtenv/lib/python3.9/site-packages/nibabel/tests/data/tinypet.v'>

Author: DimitriPapadopoulos

nibabel/tests/test_ecat.py

@@ -58,7 +58,7 @@ def test_dtype(self):
             hdr.get_data_dtype()
 
     def test_header_codes(self):
-        fid = open(ecat_file, 'rb')
+        fid = open(self.example_file, 'rb')
         hdr = self.header_class()
         newhdr = hdr.from_fileobj(fid)
         fid.close()
@@ -119,6 +119,7 @@ def test_mlist_errors(self):
         hdr = self.header_class.from_fileobj(fid)
         hdr['num_frames'] = 6
         mlist = read_mlist(fid, hdr.endianness)
+        fid.close()
         mlist = np.array([[1.68427540e+07, 3.00000000e+00,
                            1.20350000e+04, 1.00000000e+00],
                           [1.68427530e+07, 1.20360000e+04,