tests/plugins: replace rand_ish use with a counter
tests/plugins: replace rand_ish use with a counter

Rather than generating a random temporary file name for each plugin
generated by the test, just use a counter to ensure uniqueness.

This was the only remaining rand_ish user, so this change will allow the
removal of rand_ish.

BUG=None
TEST=cargo test --features=plugin

Change-Id: I8d42da97870db54a850f686826ca107255b8c3ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3017608
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm