Instead of debugging all that, here's my SSCCE showing --installdeps happily trying to install the test requirements. I've intentionally used deps which don't exist.

$ ls
cpanfile
$ cat cpanfile
on 'test' => sub {
    requires 'Nosuch', '1.0';
    suggests 'Stillno', '1.0';
};
$ cpanm -S --installdeps .
--> Working on .
Configuring /tmp/testreq ... OK
==> Found dependencies: Nosuch
! Finding Nosuch (1.0) on mirror https://cpan.metacpan.org failed.
! Couldn't find module or a distribution Nosuch (1.0)
! Installing the dependencies failed: Module 'Nosuch' is not installed
! Bailing out the installation for ..
$
[download]

Hopefully you can use this as a starting point to see why your deps are being ignored.