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

test/unit/main.test.js

@@ -311,13 +311,6 @@ for (const { history, latencies, statistics } of [
         latencies: [10, 40, 20, 25],
         statistics: { min: 10, max: 40, average: 24, median: 23 },
     },
-    {
-        // Sorted as text rather than as numbers, these answers would put
-        // 100 in the middle. The average, 39.67, is rounded.
-        history: "answers with different numbers of digits",
-        latencies: [100, 9, 10],
-        statistics: { min: 9, max: 100, average: 40, median: 10 },
-    },
 ]) {
     test(`a target's min, max, average and median latency over ${history}`, () => {
         const host = hostAfter(latencies);