patch 8.0.0552: toupper and tolower don't work properly for Turkish

Problem:    Toupper and tolower don't work properly for Turkish when 'casemap'
            is empty. (Bjorn Linse)
Solution:   Check the 'casemap' options when deciding how to upper/lower case.

src/testdir/test_normal.vim

@@ -1603,6 +1603,26 @@ fun! Test_normal30_changecase()
   norm! V~
   call assert_equal('THIS IS A simple test: äüöss', getline('.'))
 
+  " Turkish ASCII turns to multi-byte.
+  try
+    lang tr_TR.UTF-8
+    set casemap=
+    call setline(1, 'iI')
+    1normal gUU
+    call assert_equal("\u0130I", getline(1))
+    call assert_equal("\u0130I", toupper("iI"))
+
+    call setline(1, 'iI')
+    1normal guu
+    call assert_equal("i\u0131", getline(1))
+    call assert_equal("i\u0131", tolower("iI"))
+
+    lang en_US.UTF-8
+  catch /E197:/
+    " can't use Turkish locale
+    throw 'Skipped: Turkish locale not available'
+  endtry
+
   " clean up
   bw!
 endfunc