gdb.base/default.exp: Add missing $_as_string to "show convenience" test

gdb.base/default.exp: Add missing $_as_string to "show convenience" test

When adding the $_as_string convenience function, I missed a new test
failure in default.exp.  The tests lists the convenience functions, so
$_as_string should be added to the expected list.

Fixes:

+FAIL: gdb.base/default.exp: show convenience ($_caller_is = <internal function _caller_is> not found)

gdb/testsuite/ChangeLog:

	* gdb.base/default.exp: Add $_as_string to the list of expected
	convenience functions.