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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.76%. Comparing base (37dcd06) to head (54c855c).
Report is 90 commits behind head on main.

Additional details and impacted files

@@           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	Coverage Δ
provider-base	24.71% <100.00%> (ø)

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[Hook25]

+1, lgtm