Fix think-o in calls to gdb_compile.

2015-11-19  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/testsuite/
	* gdb.base/nested-subp1.exp: Pass executable, not executable name,
	as type argument to gdb_compile.
	* gdb.base/nested-subp2.exp: Likewise.
	* gdb.base/nested-subp3.exp: Likewise.