[DSLX/IR] `xls_dslx_test` doesn't compare interpreter results with JIT

PR #958 allowed for `compare=none` to be removed from `xls_dslx_test`s that are using test_proc.
One of such designs is `matmul_4x4`. `matmul_4x4_dslx_test` target is successful, but it shouldn't be.

IR generated from `matmul_4x4` is empty:
```
package matmul_4x4

```
JIT and interpreter results should be different.

/cc @proppy