test-bheads: use a case-insensitive set of filenames (issue2274)

This prevents false negatives on case-insensitive systems.

tests/test-bheads

@@ -137,21 +137,21 b' hg ci -m "b3 (HN)"'
 echo '% case NN: msg'
 hg up -q null
 hg branch -f b
-echo 1 > B
+echo 1 > bb
 hg ci -Am "b4 (NN): new topo root for branch b"
 
 echo '% case HN: no msg'
-echo 2 >> B
+echo 2 >> bb
 hg ci -m "b5 (HN)"
 
 echo '% case BN: msg'
 hg branch -f default
-echo 1 > A
+echo 1 > aa
 hg ci -Am "a6 (BN): new branch root"
 
 echo '% case CN: msg'
 hg up -q 4
-echo 3 >> BB
+echo 3 >> bbb
 hg ci -Am "b7 (CN): regular new head"
 
 echo '% case BB: msg'

tests/test-bheads.out

@@ -187,15 +187,15 b' marked working directory as branch b'
 adding b
 % case NN: msg
 marked working directory as branch b
-adding B
+adding bb
 created new head
 % case HN: no msg
 % case BN: msg
 marked working directory as branch default
-adding A
+adding aa
 created new head
 % case CN: msg
-adding BB
+adding bbb
 created new head
 % case BB: msg
 marked working directory as branch default