Add test helpers

This makes waiting for text robust.

script.addPreloadScript() doesn't export the declared functions, so we
need to attach them to `window`. We also don't need all of them.

test-functions.js

@@ -0,0 +1,75 @@
+window.ph_select = function(sel) {
+    if (!window.Sizzle) {
+        return Array.from(document.querySelectorAll(sel));
+    }
+
+    if (sel.includes(":contains(")) {
+        if (!window.Sizzle) {
+            throw new Error("Using ':contains' when window.Sizzle is not available.");
+        }
+        return window.Sizzle(sel);
+    } else {
+        return Array.from(document.querySelectorAll(sel));
+    }
+};
+
+window.ph_find = function(sel) {
+    const els = window.ph_select(sel);
+    if (els.length === 0)
+        throw new Error(sel + " not found");
+    if (els.length > 1)
+        throw new Error(sel + " is ambiguous");
+    return els[0];
+};
+
+window.ph_text = function(sel) {
+    const el = window.ph_find(sel);
+    if (el.textContent === undefined)
+        throw new Error(sel + " can not have text");
+    // 0xa0 is a non-breakable space, which is a rendering detail of Chromium
+    // and awkward to handle in tests; turn it into normal spaces
+    return el.textContent.replaceAll("\xa0", " ");
+};
+
+window.ph_is_visible = function(sel) {
+    const el = window.ph_find(sel);
+    return el.tagName === "svg" || ((el.offsetWidth > 0 || el.offsetHeight > 0) && !(el.style.visibility === "hidden" || el.style.display === "none"));
+};
+
+class PhWaitCondTimeout extends Error {
+    constructor(description) {
+        if (description && description.apply)
+            description = description.apply();
+        if (description)
+            super(description);
+        else
+            super("condition did not become true");
+    }
+}
+
+window.ph_wait_cond = function (cond, timeout, error_description) {
+    return new Promise((resolve, reject) => {
+        // poll every 100 ms for now;  FIXME: poll less often and re-check on mutations using
+        // https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
+        let stepTimer = null;
+        let last_err = null;
+        const tm = window.setTimeout(() => {
+            if (stepTimer)
+                window.clearTimeout(stepTimer);
+            reject(last_err || new PhWaitCondTimeout(error_description));
+        }, timeout);
+        function step() {
+            try {
+                if (cond()) {
+                    window.clearTimeout(tm);
+                    resolve();
+                    return;
+                }
+            } catch (err) {
+                last_err = err;
+            }
+            stepTimer = window.setTimeout(step, 100);
+        }
+        step();
+    });
+};