Use mi_varobj_update in mi-var-child.exp and mi2-var-child.exp

Use mi_varobj_update in mi-var-child.exp and mi2-var-child.exp

Hi,
I modify mi-var-child.exp and find that the pattern to match the output
of -var-update * is quite complicated.  However, it can be simplified by
using mi_varobj_update.  That is what this patch does.

gdb/testsuite:

2014-10-14  Yao Qi  <yao@codesourcery.com>

	* gdb.mi/mi-var-child.exp: Use mi_varobj_update to simplify
	tests.
	* gdb.mi/mi2-var-child.exp: Likewise.