testing: extend tests for register allocator

[mmcloughlin]

We are reliant primarily on the examples for testing at this point. It would be good to "stress" the allocator because I would be (pleasantly) surprised if it's bug-free.

• More extensive unit testing
• Integration test under tests/ that (for example) uses the max number of registers of a given kind and confirms the register allocator doesn't fall over
• More specifically, handling of "casting" is a little ugly. It would be good to have a test that stresses this specifically. Perhaps, something that allocates 16 virtual general purpose registers and then accesses all sub-registers. This is clearly a possible allocation, but I could imagine the allocator messing up.
• I'm sure other tests will come to mind

Unit tests right now are trivial:

avo/pass/alloc_test.go

Lines 9 to 46 in 7752262

	func TestAllocatorSimple(t *testing.T) {
	c := reg.NewCollection()
	x, y := c.XMM(), c.YMM()
	a, err := NewAllocatorForKind(reg.KindVector)
	if err != nil {
	t.Fatal(err)
	}
	a.Add(x)
	a.Add(y)
	a.AddInterference(x, y)
	alloc, err := a.Allocate()
	if err != nil {
	t.Fatal(err)
	}
	t.Log(alloc)
	if alloc[x] != reg.X0 || alloc[y] != reg.Y1 {
	t.Fatalf("unexpected allocation")
	}
	}
	func TestAllocatorImpossible(t *testing.T) {
	a, err := NewAllocatorForKind(reg.KindVector)
	if err != nil {
	t.Fatal(err)
	}
	a.AddInterference(reg.X7, reg.Z7)
	_, err = a.Allocate()
	if err == nil {
	t.Fatal("expected allocation error")
	}
	}

[mmcloughlin]

See #65 for an apparent problem.