Frontend unit tests cover durations, colours, statistics and health (closes #21)

New table-driven tests in test/unit/main.test.js: humanDuration, the
figure and sparkline colours either side of each latency boundary, a
target's min, max, average and median over an empty history, an
all-unreachable one, a mixed one and one of only answers, and the four
health states either side of their thresholds, with targets found
unreachable counted as timed out. src/main.js exports the four names
they need.

package.json gains a test script, which script/frontend-test runs with
the dot reporter and, if a test fails, again with the spec reporter
before failing. NODE_OPTIONS picks the reporter, since yarn appends its
arguments after the test files.

Model: opus-5-5

script/frontend-test

@@ -1,14 +1,22 @@
 #!/bin/sh
 # script/frontend-test: run the frontend test suite: the unit tests in
-# test/unit/ with Node's built-in test runner, then the production
-# build, which fails on broken code.
+# test/unit/, through the test script in package.json, then the
+# production build, which fails on broken code. The tests print a dot
+# each; if any fails, they run again with every test listed, and the
+# script fails even if that run passes. NODE_OPTIONS chooses the
+# reporter because yarn adds its arguments after the test files, where
+# node would take a reporter option for one more file.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 30 node --test test/unit/*.test.js
+    NODE_OPTIONS=--test-reporter=dot timeout 30 yarn --silent run test || {
+        echo "--- Rerunning with every test listed for details ---"
+        NODE_OPTIONS=--test-reporter=spec timeout 30 yarn --silent run test
+        exit 1
+    }
     timeout 30 yarn build
 }