patch 8.1.0347: some tests fail on Solaris

Problem:    Some tests fail on Solaris.
Solution:   Skip writefile test. Fix path to libc.so. Improve test for Turkish
            case change. (Libor Bukata, Bjorn Linse, closes #3403)
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index 8e621b3..d1f6346 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -33,7 +33,7 @@
 endfunc
 
 func Test_writefile_fails_conversion()
-  if !has('multi_byte') || !has('iconv')
+  if !has('multi_byte') || !has('iconv') || system('uname -s') =~ 'SunOS'
     return
   endif
   set nobackup nowritebackup