t/unit-tests: add tests for the in-memory object source #27806

Job	Run time
config	7s
fuzz smoke test	1m 8s
sparse	3m 16s
win build	4m 56s
win+Meson build	3m 58s
rust-analysis	1m 29s
static-analysis	5m 24s
documentation	7m 28s
win+VS build	0s
osx-meson (macos-14)	25m 24s
osx-clang (macos-14)	16m 20s
osx-reftable (macos-14)	23m 45s
osx-gcc (macos-14)	19m 23s
linux-breaking-changes (ubuntu:rolling)	1m 40s
linux-TEST-vars (ubuntu:20.04)	13m 5s
linux-meson (ubuntu:rolling)	1m 13s
linux-leaks (ubuntu:rolling)	1m 3s
fedora-breaking-changes-meson (fedora:latest)	57s
linux-asan-ubsan (ubuntu:rolling)	1m 18s
debian-11 (debian:11)	10m 0s
linux-musl-meson (alpine:latest)	6m 44s
linux-sha256 (ubuntu:rolling)	1m 14s
linux-reftable (ubuntu:rolling)	1m 16s
almalinux-8 (almalinux:8)	10m 30s
linux-reftable-leaks (ubuntu:rolling)	2m 19s
linux32 (i386/ubuntu:focal)	10m 31s
win+VS test	0s
win+Meson test (0)	9m 35s
win+Meson test (4)	7m 17s
win+Meson test (1)	11m 1s
win+Meson test (6)	8m 45s
win+Meson test (8)	9m 5s
win+Meson test (3)	6m 47s
win+Meson test (2)	10m 11s
win+Meson test (9)	7m 54s
win+Meson test (7)	7m 7s
win+Meson test (5)	9m 30s
win test (5)	8m 48s
win test (2)	8m 33s
win test (0)	7m 9s
win test (3)	8m 57s
win test (8)	6m 2s
win test (4)	7m 42s
win test (1)	7m 51s
win test (6)	7m 2s
win test (7)	5m 51s
win test (9)	11m 17s
	5h 40m 52s

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