tests: make directory to prevent test process from going out of $TESTTMP

Before this patch, test process for test-bookmarks.t goes out of
$TESTTMP at "cd .." before creation of "orderrepo" repository.

To prevent test process from going out of $TESTTMP, this patch makes
directory "repo" sub-directory and executes almost all test scenarios
in test-bookmarks.t under it.

This is preparation for new test added in subsequent patch.

tests/test-bookmarks.t

@@ -1,4 +1,5 b''
-  $ hg init
+  $ hg init repo
+  $ cd repo
 
 no bookmarks
 
@@ -630,7 +631,7 b' We warn about divergent during bare upda'
      Z                         2:db815d6d32e6
      x  y                      2:db815d6d32e6
   $ hg -R ../cloned-bookmarks-manual-update-with-divergence pull
-  pulling from $TESTTMP
+  pulling from $TESTTMP/repo (glob)
   searching for changes
   adding changesets
   adding manifests