Prevent make test from polluting ~/.opencodereview/sessions/ with
thousands of var-folders-* directories by redirecting test session
writes to ~/.opencodereview/test-sessions/.
Introduce a package-level sessionSubDir variable (default "sessions")
and an exported UseTestSessions() function that switches it to
"test-sessions". Each test package that creates sessions calls
UseTestSessions() from init() in an init_test.go file.