test-mq-symlinks: fix symlink handling on solaris

It seems like ln -s gets confused if an existing symlink is dangling:
$ rm -f a b; ln -s a b; ln -sf b b
ln: cannot create b: File exists

We now rely on rm instead of on ln -sf.

tests/test-mq-symlinks

@@ -61,6 +61,7 b' ln -s linka linka'
 hg add linka
 hg qnew link
 hg mv linka linkb
+rm linkb
 ln -sf linkb linkb
 hg qnew movelink
 hg qpop
@@ -71,4 +72,4 b' hg qpop'
 ln -s linkbb linkb
 hg qpush
 
-true
  No newline at end of file
+true