shell/lib,veyron/tools/vrun: have vrun's test.sh use a test mounttable

Currently, vrun does not invoke setup_server_test (because it doesn't want the
credentials management that comes with that), but as a side-effect, it missed
out on the test mounttable (so it ends up using dev.v.io:8101).

This change refactors the mounttable setup part in shell_test.sh so it can be
invoked independently, and changes vrun/test.sh to use it (thereby breaking the
dependency on dev.v.io).

Change-Id: Iacd5672cfe1c746d1c97f6072d8a5dfaed639bf3
diff --git a/tools/vrun/testchild.sh b/tools/vrun/testchild.sh
index f6ec478..916ae52 100755
--- a/tools/vrun/testchild.sh
+++ b/tools/vrun/testchild.sh
@@ -5,6 +5,8 @@
 source "$(go list -f {{.Dir}} v.io/core/shell/lib)/shell_test.sh"
 
 main() {
+  shell_test::setup_mounttable
+
   local -r PINGPONG="$(shell_test::build_go_binary 'v.io/core/veyron/security/agent/pingpong')"
   local -r VRUN="$(shell_test::build_go_binary 'v.io/core/veyron/tools/vrun')"
   local -r PRINCIPAL="$(shell_test::build_go_binary 'v.io/core/veyron/tools/principal')"