tests: add StructEqual() for verifying Equal method of structs

[shoenig]

This PR adds StructEqual which is helpful for verifying that modifying
the fields of a struct will cause its Equal method of the struct to
no longer return true.

e.g. usage

func TestAffinity_Equal(t *testing.T) {
	must.Equal[*Affinity](t, nil, nil)
	must.NotEqual[*Affinity](t, nil, new(Affinity))
	must.StructEqual(t, &Affinity{
		LTarget: "left",
		RTarget: "right",
		Operand: "op",
		Weight:  100,
	}, []must.Tweak[*Affinity]{{
		Field: "LTarget",
		Apply: func(a *Affinity) { a.LTarget = "left2" },
	}, {
		Field: "RTarget",
		Apply: func(a *Affinity) { a.RTarget = "right2" },
	}, {
		Field: "Operand",
		Apply: func(a *Affinity) { a.Operand = "op2" },
	}, {
		Field: "Weight",
		Apply: func(a *Affinity) { a.Weight = 50 },
	}})
}