test: correctly handle installation of newly introduced binaries

In certain cases we might attempt to install a binary which is already
present in the test image, yet it's missing from the host system.
In such cases, let's check if the binary indeed exists in the image
before doing any other chcecks. If it does, immediately return with
success.

This was discovered during installation of
/usr/lib/systemd/systemd-bless-boot, which was not present in Ubuntu CI
(as the installed systemd was from the Ubuntu repositories), and the
binary itself was already in the image thanks to `ninja install`.
However, during extraction of binaries from the systemd service files,
another attempt to install this binary was made, which failed due to
`find_binary` being unable to find it.