test: correctly process multiline strings in $KERNEL_APPEND

Some tests (like TEST-02) set a multiline string to $KERNEL_APPEND
(which is a valid thing to do), unfortunately we'd use only the first
line of it and throw the rest away, e.g:

```
$ printf "%s" "$x"
hello

this is a multiline

kernel command line
$ read -ra out <<< "$x"
$ printf "%s" "${out[@]}"
hello
```

Let's use readarray/mapfile instead to avoid this:

```
$ readarray out <<< "$x"
$ printf "%s" "${out[@]}"
hello

this is a multiline

kernel command line

```

test/test-functions

@@ -498,7 +498,7 @@ run_qemu() {
 
     if [[ -n "${KERNEL_APPEND:=}" ]]; then
         local user_kernel_append
-        read -ra user_kernel_append <<< "$KERNEL_APPEND"
+        readarray user_kernel_append <<< "$KERNEL_APPEND"
         kernel_params+=("${user_kernel_append[@]}")
     fi
 
@@ -557,7 +557,7 @@ run_nspawn() {
 
     if [[ -n "${KERNEL_APPEND:=}" ]]; then
         local user_kernel_append
-        read -ra user_kernel_append <<< "$KERNEL_APPEND"
+        readarray user_kernel_append <<< "$KERNEL_APPEND"
         kernel_params+=("${user_kernel_append[@]}")
     fi