test: configure ldconfig's cache in the minimal verity images

The glibc stuff on ppc64le C8S is a little bit wild, as there are two
versions:

```
$ ldconfig -p | grep libc.so
        libc.so.6 (libc6,64bit, hwcap: "power9", OS ABI: Linux 3.10.0) => /lib64/glibc-hwcaps/power9/libc-2.28.so
        libc.so.6 (libc6,64bit, OS ABI: Linux 3.10.0) => /lib64/libc.so.6
```

and with `/etc/ld.so.cache` present all binaries use the first one:

```
$ ldd /bin/cat
        linux-vdso64.so.1 (0x00007fffa8070000)
        libc.so.6 => /lib64/glibc-hwcaps/power9/libc-2.28.so (0x00007fffa7e20000)
        /lib64/ld64.so.2 (0x00007fffa8090000)
```

However, without the cache the binaries will fall back to `/lib64/libc.so.6`
which breaks tests that use the minimal verity images (like TEST-29),
because we install only the first version (that's shown by `ldd` at
the time the images are created):

```
[   91.595343] testsuite-29.sh[747]: + portablectl --profile=trusted attach --now --runtime /usr/share/minimal_0.raw minimal-app0
         Starting systemd-portabled.service...
[  OK  ] Started systemd-portabled.service.
         Starting minimal-app0-foo.service...
         Starting minimal-app0.service...
[  104.432217] cat[858]: cat: error while loading shared libraries: libc.so.6: cannot open shared object file: No such file or directory
[  104.435080] cat[857]: cat: error while loading shared libraries: libc.so.6: cannot open shared object file: No such file or directory
[FAILED] Failed to start minimal-app0.service.
See 'systemctl status minimal-app0.service' for details.
```

```
$ chroot /var/tmp/systemd-test.nMHPfc/minimal/
/bin/bash: error while loading shared libraries: libc.so.6: cannot open shared object file: No such file or directory
```

With the ldconfig's cache it seems to work as expected:
```
$ chroot /var/tmp/systemd-test.gVtYLg/minimal
bash-4.4# cat --version
cat (GNU coreutils) 8.30
...
```

test/test-functions

@@ -630,6 +630,7 @@ install_verity_minimal() {
         mkdir -p "$initdir/usr/lib/systemd/system" "$initdir/usr/lib/extension-release.d" "$initdir/etc" "$initdir/var/tmp" "$initdir/opt" "$initdir/var/lib/app1"
         setup_basic_dirs
         install_basic_tools
+        install_ld_so_conf
         # Shellcheck treats [[ -v VAR ]] as an assignment to avoid a different
         # issue, thus falsely triggering SC2030 in this case
         # See: koalaman/shellcheck#1409