selftest: remove "Yoda ordering" in assertions

selftest: remove "Yoda ordering" in assertions

gcc/ChangeLog:
	* selftest.c (assert_streq): Rename "expected" and "actual" to
	"val1" and "val2".  Extend NULL-handling to cover both inputs
	symmetrically, while still requiring both to be non-NULL for a pass.
	* selftest.h (assert_streq): Rename "expected" and "actual" to
	"val1" and "val2".
	(ASSERT_EQ): Likewise.
	(ASSERT_EQ_AT): Likewise.
	(ASSERT_KNOWN_EQ): Likewise.
	(ASSERT_KNOWN_EQ_AT): Likewise.
	(ASSERT_NE): Likewise.
	(ASSERT_MAYBE_NE): Likewise.
	(ASSERT_MAYBE_NE_AT): Likewise.
	(ASSERT_STREQ): Likewise.  Clarify that both must be non-NULL for
	the assertion to pass.
	(ASSERT_STREQ_AT): Likewise.

From-SVN: r259782