Fix missing `subprocess.PIPE` in reboot_check_test.py (Bugfix)

[tomli380576]

Description

The new reboot check test was missing stdout=subprocess.PIPE in calls to subprocess.run which was causing it to return None for <CompletedProcess>.stdout

Resolved issues

1548

Documentation

Tests

1. The original test cases pass
2. Side loaded on 202406-34083 and full wb/cb tests can be run

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 47.76%. Comparing base (37dcd06) to head (54c855c). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1546 +/- ## ======================================= Coverage 47.76% 47.76% ======================================= Files 370 370 Lines 39750 39750 Branches 6720 6720 ======================================= Hits 18987 18987 Misses 20048 20048 Partials 715 715 ``` | [Flag](https://app.codecov.io/gh/canonical/checkbox/pull/1546/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | Coverage Δ | | |---|---|---| | [provider-base](https://app.codecov.io/gh/canonical/checkbox/pull/1546/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | `24.71% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical#carryforward-flags-in-the-pull-request-comment) to find out more.

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