test-casefolding: fix wrong case on hfs on linux

on linux hfs keeps the old case of the file stored
force it by renameing to another name and back

tests/test-casefolding

@@ -31,6 +31,9 b' hg ci -Am adda'
 hg rm a
 hg ci -Am removea
 echo A > A
+# on linux hfs keeps the old case stored, force it
+mv a aa
+mv aa A
 hg ci -Am addA
 # Used to fail under case insensitive fs
 hg up -C 0