Fix READ(6) tests' capacity checks

test_read6_simple.c does a check on LUN size:

```C
	if (num_blocks >= 0x1fffff) {
		CU_PASS("LUN is too big for read-beyond-eol tests with READ6. Skipping test.\n");
		return;
	}
```

but if I understand correctly, a 1 GiB volume should have exactly 0x200000 blocks. The read-beyond-eol test also has a similar check that would skip the rest of the test if the volume has exactly 0x1fffff blocks. In this case, I believe it should run...

Patch attached.