acceptance: a later FAIL blocks inheritance, ffboot is a component, the units fallback is valid

The inheritance walk skipped every record that was not a PASS on the
current fingerprint, so a FAIL or ERROR recorded after a PASS on the
same image was stepped over and the older PASS inherited into the next
campaign. The newest record on the fingerprint now decides: a PASS is
inherited, a FAIL or ERROR blocks it (reason failed-since), an ABORTED
run says nothing. Unit tests for all three orders.

ffboot, the tool that rewrites the boot environment on every install and
slot switch, was packaged from scripts/ outside every fingerprint. It
now lives in the recipe's files and the recipe inherits the manifest
class; the tree manifest tool fingerprints file components the same
way, and the update tests cover the component.

forgectrl.settings-bounds fell back to ui_units=mm, which the whitelist
refuses, so the always-required test failed on a fresh machine; the
fallback is metric.

forgetest/forgetest/suite/forgectrl.py

@@ -165,7 +165,7 @@ def settings_bounds(ctx):
             key = k
             break
     if key is None:
-        key, val = "ui_units", "mm"
+        key, val = "ui_units", "metric"
         ctx.log("no settable key is present; writing %s=%s (recorded in evidence)", key, val)
     else:
         val = before[key]

forgetest/forgetest/suite/update.py

@@ -5,7 +5,8 @@ import tempfile
 from ..catalog import test
 from .. import hw
 
-_UPDATE_COVERS = [("forgectrl", "src/update.c"), ("forgectrl", "src/update.h")]
+_UPDATE_COVERS = [("forgectrl", "src/update.c"), ("forgectrl", "src/update.h"),
+                  ("ffboot", "**")]
 
 
 @test("update.slots-and-signature", title="Boot slots readable, unsigned/tampered archives refused",