icasefs: add test for case preservation on case insensitive filesystem

"hg qpush" causes unexpected behavior, if case preservation on case
insensitive filesystem is not enough.

this patch adds the test using mixed-case filenames to reproduce this
problem on any case insensitive filesystems.

tests/test-mq.t

@@ -1394,3 +1394,46 b' test popping must remove files added in '
   patch queue now empty
   $ cd ..
 
+
+test case preservation through patch pushing especially on case
+insensitive filesystem
+
+  $ hg init casepreserve
+  $ cd casepreserve
+
+  $ hg qnew add-file1
+  $ echo a > TeXtFiLe.TxT
+  $ hg add TeXtFiLe.TxT
+  $ hg qrefresh
+
+  $ hg qnew add-file2
+  $ echo b > AnOtHeRFiLe.TxT
+  $ hg add AnOtHeRFiLe.TxT
+  $ hg qrefresh
+
+  $ hg qnew modify-file
+  $ echo c >> AnOtHeRFiLe.TxT
+  $ hg qrefresh
+
+  $ hg qapplied
+  add-file1
+  add-file2
+  modify-file
+  $ hg qpop -a
+  popping modify-file
+  popping add-file2
+  popping add-file1
+  patch queue now empty
+
+this qpush causes problems below, if case preservation on case
+insensitive filesystem is not enough:
+(1) unexpected "adding ..." messages are shown
+(2) patching fails in modification of (1) files
+
+  $ hg qpush -a
+  applying add-file1
+  applying add-file2
+  applying modify-file
+  now at: modify-file
+
+  $ cd ..