tests: update the detailed exit codes for icasefs gated tests

The fact that there's already a detailed exit code after the last change here in
test-casecollision-merge.t gives me some pause, but maybe it was found and
changed manually?

Differential Revision: https://phab.mercurial-scm.org/D10298

tests/test-casecollision-merge.t

@@ -145,7 +145,7 b" target file is not 'a'/'A' but 'b'/'B' i"
 
   $ hg merge
   abort: case-folding collision between [aA] and [Aa] (re)
-  [255]
+  [20]
   $ hg parents --template '{rev}\n'
   4
   $ hg status -A
@@ -158,7 +158,7 b" target file is not 'a'/'A' but 'b'/'B' i"
   1 files updated, 0 files merged, 2 files removed, 0 files unresolved
   $ hg merge
   abort: case-folding collision between [aA] and [Aa] (re)
-  [255]
+  [20]
   $ hg parents --template '{rev}\n'
   2
   $ hg status -A
@@ -213,7 +213,7 b' Directory/file case-folding collision:'
 
   $ hg merge 0
   abort: case-folding collision between Aa and directory of aA/a
-  [255]
+  [20]
 (note: no collision between 0 and 00 or 000/f)
 
 Directory case-folding collision:
@@ -328,7 +328,7 b' test for rename awareness of case-foldin'
   A B
   $ hg update
   abort: case-folding collision between [bB] and [Bb] (re)
-  [255]
+  [20]
 
   $ hg update --check
   abort: uncommitted changes

tests/test-casefolding.t

@@ -115,7 +115,7 b' no clobbering of untracked files with wr'
   $ hg up
   A: untracked file differs
   abort: untracked files in working directory differ from files in requested revision
-  [255]
+  [20]
   $ cat a
   gold
   $ rm a