kunit: factor out str constants from binary assertion structs

kunit: factor out str constants from binary assertion structs

If the compiler doesn't optimize them away, each kunit assertion (use of
KUNIT_EXPECT_EQ, etc.) can use 88 bytes of stack space in the worst and
most common case. This has led to compiler warnings and a suggestion
from Linus to move data from the structs into static const's where
possible [1].

This builds upon [2] which did so for the base struct kunit_assert type.
That only reduced sizeof(struct kunit_binary_assert) from 88 to 64.

Given these are by far the most commonly used asserts, this patch
factors out the textual representations of the operands and comparator
into another static const, saving 16 more bytes.

In detail, KUNIT_EXPECT_EQ(test, 2 + 2, 5) yields the following struct
  (struct kunit_binary_assert) {
    .assert = <struct kunit_assert>,
    .operation = "==",
    .left_text = "2 + 2",
    .left_value = 4,
    .right_text = "5",
    .right_value = 5,
  }
After this change
  static const struct kunit_binary_assert_text __text = {
    .operation = "==",
    .left_text = "2 + 2",
    .right_text = "5",
  };
  (struct kunit_binary_assert) {
    .assert = <struct kunit_assert>,
    .text = &__text,
    .left_value = 4,
    .right_value = 5,
  }

This also DRYs the code a bit more since these str fields were repeated
for the string and pointer versions of kunit_binary_assert.

Note: we could name the kunit_binary_assert_text fields left/right
instead of left_text/right_text. But that would require changing the
macros a bit since they have args called "left" and "right" which would
be substituted in `.left = #left` as `.2 + 2 = \"2 + 2\"`.

[1] https://groups.google.com/g/kunit-dev/c/i3fZXgvBrfA/m/VULQg1z6BAAJ
[2] https://lore.kernel.org/linux-kselftest/20220113165931.451305-6-dlatypov@google.com/

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>