test: clarify test-parseindex offsets

We will make this revlog non-inline, so we clarify the code to make sure it is
simple to adjust the test later.

tests/test-parseindex.t

@@ -145,8 +145,13 b' Test corrupted p1/p2 fields that could c'
   > ]
   > for n, p in poisons:
   >     # corrupt p1 at rev0 and p2 at rev1
-  >     d = data[:24] + p + data[28:127 + 28] + p + data[127 + 32:]
-  >     open(n + b"/.hg/store/00changelog.i", "wb").write(d)
+  >     rev_0 = data[:64 + 63]
+  >     rev_1 = data[64 + 63:]
+  >     altered_rev_0 = rev_0[:24] + p + rev_0[24 + 4:]
+  >     altered_rev_1 = rev_1[:28] + p + rev_1[28 + 4:]
+  >     new_data = altered_rev_0 + altered_rev_1
+  >     with open(n + b"/.hg/store/00changelog.i", "wb") as f:
+  >         f.write(new_data)
   > EOF
 
   $ hg -R limit debugrevlogindex -f1 -c