pkg/uefi/meregion: add TestFindFPTSignature test case

[orangecms]

I have no idea what Go is trying to tell me:

--- FAIL: TestFindFPTSignature (0.00s)
    --- FAIL: TestFindFPTSignature/beginning (0.00s)
panic: runtime error: slice bounds out of range [:20] with capacity 16 [recovered]
        panic: runtime error: slice bounds out of range [:20] with capacity 16

goroutine 4010 [running]:
testing.tRunner.func1.2({0x5af120, 0xc00001cc90})
        /home/dama/bin/go1.21.0/src/testing/testing.go:1545 +0x238
testing.tRunner.func1()
        /home/dama/bin/go1.21.0/src/testing/testing.go:1548 +0x397
panic({0x5af120?, 0xc00001cc90?})
        /home/dama/bin/go1.21.0/src/runtime/panic.go:914 +0x21f
github.com/linuxboot/fiano/pkg/uefi.FindMEDescriptor({0xc000f09e00, 0x5ac?, 0x10})
        /home/dama/Projects/Fiedka/fiano/pkg/uefi/meregion.go:106 +0x125
github.com/linuxboot/fiano/pkg/uefi.TestFindFPTSignature.func1(0xc000bd3860)
        /home/dama/Projects/Fiedka/fiano/pkg/uefi/meregion_test.go:84 +0x34
testing.tRunner(0xc000bd3860, 0xc00002b160)
        /home/dama/bin/go1.21.0/src/testing/testing.go:1595 +0xff
created by testing.(*T).Run in goroutine 4009
        /home/dama/bin/go1.21.0/src/testing/testing.go:1648 +0x3ad
FAIL    github.com/linuxboot/fiano/pkg/uefi     0.284s
FAIL

[orangecms]

sigh I got it... there is this at the end of the row hex.Dump(buf[:20]) - it is so long though that I hadn't seen it