test: avoid double-fsck'ing of the rootfs on Arch

test: avoid double-fsck'ing of the rootfs on Arch

Since systemd 206 the combination of systemd and mkinitcpio
causes, under certain conditions, the rootfs to be double fsck'd.

Symptoms:
```
:: performing fsck on '/dev/sda1'
systemd: clean, 3523/125488 files, 141738/501760 blocks
********************** WARNING **********************
*                                                   *
*  The root device is not configured to be mounted  *
*  read-write! It may be fsck'd again later.        *
*                                                   *
*****************************************************
<snip>
[  OK  ] Started File System Check on Root Device
```

This occurs when neither 'ro' or 'rw', or only 'ro' is present
on the kernel command line. The solution is to mount the roofs
as read-write on the kernel command line, so systemd knows to not fsck
it again.