Add a persisted test suite

`bundle exec rake test` (minitest, a default gem but pinned in
Gemfile for reproducibility) now covers what ad-hoc scripts kept
re-verifying by hand during development: Gpx/Fit/TrackStats parsing,
GpxStore/LiveTrackStore file/TTL logic, LiveTrackClient's payload
parsing, and app.rb's routes end to end via Rack::MockRequest.

MAPTOOL_DATA_DIR (new, app.rb) lets the suite point GPX_STORE/
LIVETRACK_STORE at a throwaway directory instead of real data/ —
without it every run would leave uploaded test files behind. Each
lib file's old `if __FILE__ == $0` self-check is now redundant with
real tests covering the same ground, so removed rather than kept as
a second copy of the same assertions. Wired into ci.yml so it isn't
just a local convenience.

app.rb

@@ -32,9 +32,13 @@ module Maptool
     # the container) is enough — it doesn't need to survive restarts.
     ASSET_VERSION = Time.now.to_i
 
-    OFFLINE_PMTILES_PATH = File.join(__dir__, "data", "offline.pmtiles")
-    GPX_STORE = GpxStore.new(File.join(__dir__, "data", "gpx"))
-    LIVETRACK_STORE = LiveTrackStore.new(File.join(__dir__, "data", "livetrack.json"))
+    # Overridable so the test suite can point this at a throwaway directory
+    # instead of the real data/ — nothing else needs to set this.
+    DATA_DIR = ENV.fetch("MAPTOOL_DATA_DIR") { File.join(__dir__, "data") }
+
+    OFFLINE_PMTILES_PATH = File.join(DATA_DIR, "offline.pmtiles")
+    GPX_STORE = GpxStore.new(File.join(DATA_DIR, "gpx"))
+    LIVETRACK_STORE = LiveTrackStore.new(File.join(DATA_DIR, "livetrack.json"))
 
     # Shared secret for the Garmin LiveTrack webhook (a Google Apps Script
     # watching Gmail for LiveTrack emails, see docs/deployment.adoc) — not a