Use support_nested_function_tests in gdb.base/nested-subp1.exp et al

This commit updates gdb.base/nested-subp[1-3].exp to determine
whether whether nested functions are supported using the function
support_nested_function_tests.

gdb/testsuite/ChangeLog:

	* gdb.base/nested-subp1.exp: Use support_nested_function_tests.
	* gdb.base/nested-subp2.exp: Likewise.
	* gdb.base/nested-subp3.exp: Likewise.