Run dw2-var-zero-addr.exp with --readnow

Run dw2-var-zero-addr.exp with --readnow

This patch is to extend dw2-var-zero-add.exp to cover the case that
partial symtabl is not used while full symtab is used, in order to
cover the changes in patch 2/3.  This patch restarts GDB with
--readnow and does the same test again.

gdb/testsuite:

2014-09-19  Yao Qi  <yao@codesourcery.com>

	* gdb.dwarf2/dw2-var-zero-addr.exp: Move test into new proc test.
	Invoke test.  Restart GDB with --readnow and invoke test again.