[test] Fix CPU sum output label for the MCH OpenACC CUDA MPI CPPSTD test

[cosunae]

No description provided.

[jenkins-cscs]

Can I test this patch?

[codecov-io]

Codecov Report

Merging #889 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #889      +/-   ##
==========================================
+ Coverage   91.72%   91.74%   +0.01%     
==========================================
  Files          78       78              
  Lines       10308    10308              
==========================================
+ Hits         9455     9457       +2     
+ Misses        853      851       -2

Impacted Files	Coverage Δ
reframe/core/config.py	85.58% <0%> (+1.8%)	⬆️

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[codecov-io]

Codecov Report

Merging #889 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #889      +/-   ##
==========================================
+ Coverage   91.72%   91.74%   +0.01%     
==========================================
  Files          78       78              
  Lines       10308    10308              
==========================================
+ Hits         9455     9457       +2     
+ Misses        853      851       -2

Impacted Files	Coverage Δ
reframe/core/config.py	85.58% <0%> (+1.8%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e6b8e11...8623851. Read the comment docs.

[victorusu]

lgtm