test-patchbomb: ensure hg email write to stdout

With -n/--test and if the PAGER environment variable is set, 'hg email' send its
output to the user defined pager.

If the pager capture the output, the test is unable verify it.

Unsetting the PAGER environment variable force 'hg email' to write to stdout.

tests/test-patchbomb.t

@@ -28,6 +28,9 Mercurial-patchbomb/.* -> Mercurial-patc
   $ echo "[extensions]" >> $HGRCPATH
   $ echo "patchbomb=" >> $HGRCPATH
 
+Ensure hg email output is sent to stdout
+  $ unset PAGER
+
   $ hg init t
   $ cd t
   $ echo a > a