* gdb.base/dfp-exprs.exp (test_dfp_arithmetic_expressions): Add tests

	* gdb.base/dfp-exprs.exp (test_dfp_arithmetic_expressions): Add tests
	for expressions with decimal float values.
	(test_dfp_conversions): New function to test casts to and from
	decimal float types.
	Call test_dfp_conversions.
	* gdb.base/dfp-test.c (struct decstruct): Add float4 and double8
	elements.
	(main): Initialize ds.float4 and ds.double8 elements.
	* gdb.base/dfp-test.exp (d32_set_tests): Fix typo.  Adjust expect
	string to new error message.
	(d64_set_tests): Likewise.
	(d128_set_tests): Likewise.
	Add tests for expressions with decimal float variables.  Add tests for
	conversions to and from decimal float types.