mock-capsule-capture-libs: Fix a memory leak
We allocated one GFile per loop iteration, but only freed the last one.
simple-input-device: Don't leak the HID report descriptor
tests: Don't leak list of portal backends
pv-adverb: Don't leak temporary arrays used to build search paths
tests: Don't leak an error
Unimportant on their own, but they contributed to a CI failure for !576 (merged).
I'm still seeing memory leaks with Debian unstable's GLib, but I think those might actually be a GLib regression: I'll continue to look into that as a non-critical-path thing.