testing/nose: Move example scripts into the examples/ subdirectory

Keep the base $SUBJECT/$TOOL directory clean.

diff --git a/testing/nose/close_line.py b/testing/nose/exercises/close_line.py
similarity index 100%
rename from testing/nose/close_line.py
rename to testing/nose/exercises/close_line.py

diff --git a/testing/nose/mean.py b/testing/nose/exercises/mean.py
similarity index 100%
rename from testing/nose/mean.py
rename to testing/nose/exercises/mean.py

diff --git a/testing/nose/test_mean.py b/testing/nose/exercises/test_mean.py
similarity index 100%
rename from testing/nose/test_mean.py
rename to testing/nose/exercises/test_mean.py