selftest: runqemu: better check for ROOTFS: in the log

selftest: runqemu: better check for ROOTFS: in the log

* it was searching for line like this:
  ROOTFS: [/OE/build/poky/build/build-st-2023-03-20-esdk-runqemu-patch1/runqemu.RunqemuTests.test_boot_machine_ext4/build-st/tmp/deploy/images/qemux86-64/core-image-minimal-qemux86-64-20230320081121.rootfs.ext4]
  but with IMAGE_NAME_SUFFIX changed to something else than default ".rootfs"
  or with my pending changes the line looks like this:
  ROOTFS: [/OE/build/poky/build/build-st-2023-03-20-esdk-runqemu-patch2/runqemu.RunqemuTests.test_boot_machine_ext4/build-st/tmp/deploy/images/qemux86-64/core-image-minimal-qemux86-64.rootfs-20230320085744.ext4]
  and test was failing.

* Check for whole line starting with ROOTFS: and ending just with .ext4

[YOCTO #12937]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>