test: fix validation for global cache

[stasadev]

The Issue

https://github.com/ddev/ddev-pnpm/actions/runs/34456135632/job/102802836517

  Running: bats tests in ./
  1..5
  # ddev add-on get /home/runner/work/ddev-pnpm/ddev-pnpm with project test-ddev-pnpm in /home/runner/tmp/test-ddev-pnpm.vdalgg
  not ok 1 install from directory
  # (from function `assert_success' in file /home/linuxbrew/.linuxbrew/lib/bats-assert/src/assert_success.bash, line 45,
  #  from function `health_checks' in file tests/test.bats, line 82,
  #  in test file tests/test.bats, line 127)
  #   `health_checks' failed
  #
  # -- command failed --
  # status : 1
  # output : Failed to execute command `bash -c "grep -R 'is-odd' /mnt/ddev-global-cache/pnpm 2>/dev/null | grep '3.0.1'"`: exit status 1

How This PR Solves The Issue

Changes the test logic to make it more stable.

Manual Testing Instructions

ddev add-on get https://github.com/ddev/ddev-pnpm/tarball/refs/pull/18/head
ddev restart

Automated Testing Overview

Release/Deployment Notes