*/tests: Use defer instead of finally

try/finally was used in a couple of places to save/restore default ZBlk
format setting. Move the restore part close to save with the help of
defer.

bigfile/tests/test_filezodb.py

@@ -730,8 +730,10 @@ def test_bigfile_filezodb_fmt_change():
 
     # save/restore original ZBlk_fmt_write
     fmt_write_save = file_zodb.ZBlk_fmt_write
+    def _():
+        file_zodb.ZBlk_fmt_write = fmt_write_save
+    defer(_)
 
-    try:
     # check all combinations of format pairs via working with blk #0 and
     # checking internal f structure
     for src_fmt, src_type in ZBlk_fmt_registry.items():
@@ -751,22 +753,17 @@ def test_bigfile_filezodb_fmt_change():
 
             assert type(f.blktab[0]) is dst_type
 
-    finally:
-        file_zodb.ZBlk_fmt_write = fmt_write_save
-
 
 # test that ZData are reused for changed chunks in ZBlk1 format
+@func
 def test_bigfile_zblk1_zdata_reuse():
     # set ZBlk_fmt_write to ZBlk1 for this test
     fmt_write_save = file_zodb.ZBlk_fmt_write
     file_zodb.ZBlk_fmt_write = 'ZBlk1'
-    try:
-        _test_bigfile_zblk1_zdata_reuse()
-    finally:
+    def _():
         file_zodb.ZBlk_fmt_write = fmt_write_save
+    defer(_)
 
-@func
-def _test_bigfile_zblk1_zdata_reuse():
     root = dbopen()
     defer(lambda: dbclose(root))
     root['zfile6'] = f = ZBigFile(blksize)