Fixed comment length in cgroups tests.

This style issue started creeping up in other RR's Review bot feed back:

Error:
 Checking 426 files using filter --filter=-,+build/class,
+build/deprecated,+build/endif_comment,+runtime/vlog,
+whitespace/comma,+whitespace/ending_newline,+whitespace/forcolon,
+whitespace/indent,+whitespace/line_length,+whitespace/tab,
+whitespace/todo
src/tests/cgroups_tests.cpp:829:  Lines should be <= 80 characters
long  [whitespace/line_length] [2]
Total errors found: 1

Review: https://reviews.apache.org/r/21098