tests/test-run-tests.sh: Test the helper functions used by the tests
tests/test-run-tests.sh: Test the helper functions used by the tests

This file was missing from commit 56aa1db9 (`test_killtree`, defined
here, was used by `test_runner_killtree` in 56aa1db9).

wmanley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester