Skip floating point tests in return-nodebug.exp if gdb_skip_float_test is true

return-nodebug.exp does the test for various types, but we shouldn't
test with floating point type if gdb_skip_float_test returns true.

gdb/testsuite:

2016-09-02  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/return-nodebug.exp: Skip the test if	skip_float_test
	is true and $type is "float" or "double".