tcl_tests: Abbreviate output when run under CI

To make CI output not too long - output only header of every test set
and output only relevant log entries on test failures.

When `CI' env is defined. Which should be defined for both Travis and
Cirrus. Reference:
  https://docs.travis-ci.com/user/environment-variables/
  https://cirrus-ci.org/guide/writing-tasks/#environment-variables