Fix intermittent profile.d/ scripts test failure from extra trailing newline

[edmorley]

The profile.d/ scripts test asserts on captured run_multi command output using an exact match against a heredoc. The platform log stream that captures the output occasionally appends an extra trailing newline, which intermittently breaks these exact assertions.

For example:
https://github.com/heroku/heroku-buildpack-python/actions/runs/27144980449/job/80156740097?pr=2099#step:5:628

Failures:

  1) .profile.d/ scripts sets the required run-time env vars
     Failure/Error:
       expect(output).to eq(<<~OUTPUT)
         Python buildpack: Detected 512 MB available memory and 8 CPU cores.
         Python buildpack: Skipping automatic configuration of WEB_CONCURRENCY since it's already set.
         DYNO_RAM=512
         WEB_CONCURRENCY=999
       OUTPUT

       expected: "Python buildpack: Detected 512 MB available memory and 8 CPU cores.\nPython buildpack: Skipping auto...matic configuration of WEB_CONCURRENCY since it's already set.\nDYNO_RAM=512\nWEB_CONCURRENCY=999\n"
            got: "Python buildpack: Detected 512 MB available memory and 8 CPU cores.\nPython buildpack: Skipping auto...tic configuration of WEB_CONCURRENCY since it's already set.\nDYNO_RAM=512\nWEB_CONCURRENCY=999\n\n"

       (compared using ==)
     # ./spec/hatchet/profile_d_scripts_spec.rb:128:in 'block (4 levels) in <top (required)>'
     # ./vendor/bundle/ruby/4.0.0/gems/heroku_hatchet-8.0.6/lib/hatchet/app.rb:255:in 'block in Hatchet::App#run_multi'

Adds a normalize_trailing_newlines spec helper that collapses any trailing newlines down to a single one, and applies it to the exact-match assertions in the test. This absorbs the spurious trailing newline whilst still asserting on the rest of the output exactly.

GUS-W-22900269.