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, one of only answers and one whose
answers have different numbers of digits, 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
 }