Unify incremental test cases and disable many

The self-hosted aarch64 backend is not currently functional due to the
Liveness changes. A previous commit disabled aarch64 on the behavior
tests; this commit disables it and arm for the test cases. Moreover, all
incremental test cases have been unified into shared cross-platform
cases, which can be gradually enabled as the backends improve.

test/cases/assert_function.6.zig

@@ -0,0 +1,9 @@
+pub fn main() void {
+    const a: u32 = 2;
+    const b: ?u32 = a;
+    const c = b.?;
+    if (c != 2) unreachable;
+}
+
+// run
+//