test: Fix TestMetrics.testOverview for partial dd reads
test: Fix TestMetrics.testOverview for partial dd reads

In recent Fedora 31, the dd command for the memory hog tends to abort
with a partial read after ~ 20 MB. Use the `iflag=fullblock` option (as
dd suggests by itself) to avoid that.

Closes #13608
cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpituous/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit