Add testcase generator

Very simplistics - select M fields, filters on N fields, slight bias to
use values of same type of the field it's comparing against.

No segfaults yet, but one test case that generates differing output when
run against `nulls` and `nulls1`:

```
select rowid from nulls1 where binary_9 >= '56' and ts_5 < 496886400000;
```

tests/test-all

@@ -8,3 +8,4 @@ set -x
 "$here"/test-unsupported
 "$here"/test-supported
 "$here"/test-queries
+"$here"/test-random