test_cbfs: "downgrade" listing test broken again by cbfstool v4.9

Stop trying to track every small change in the output of the "cbfstool print" command, just search for some expected keywords instead.

• Around 2015 the legacy master header stopped being generated by default: https://www.coreboot.org/CBFS#FMAP https://review.coreboot.org/c/coreboot/+/10135
• In v4.7, cbfstool added the Comp(ression) column.
• v4.8 or v4.9 added a new header " FMAP REGION: COREBOOT", see new contextual diff below.
• What's next?

--- coreboot1
+++ ./coreboot2.rom
├── cbfstool {} print
│ @@ -1,4 +1,4 @@
│  FMAP REGION: COREBOOT
│  Name                           Offset     Type           Size   Comp
│ -text                           0x0        raw               446 none
│ -(empty)                        0x200      null            32152 none
│ +text                           0x0        raw               671 none
│ +(empty)                        0x300      null            31896 none