[PRIORITY] Improve tests debugging output

[apostasie]

This is a small quality of life improvement to ease debugging of tests.

When commands are unexpectedly failing, output .String() which includes the actual command and the exit code instead of .Combined() which only shows stdout+stderr.

[apostasie]

Failure is unrelated - TestTailFollowRotateLogs - which should get better with the band aid once #3178 gets merged.

@AkihiroSuda PTAL at your convenience.

Thanks.

[AkihiroSuda]

Thanks