Add a way to clear all IndexedDB databases in browser tests (emscripten-core#23408)

Without this, `browser.test_preload_caching` works once, but running it
again finds the cached data and makes it error.

Also clean up the test a little while there.

Unblocks the test for emscripten-core#23059

Author: kripken

test/test_browser.py

@@ -560,13 +560,33 @@ def test_output_file_escaping(self):
     self.compile_btest(src, ['--pre-js', data_js_file, '-o', abs_page_file, '-sFORCE_FILESYSTEM'], reporting=Reporting.JS_ONLY)
     self.run_browser(page_file, '/report_result?exit:0')
 
+  # Clear all IndexedDB databases. This gives us a fresh state for tests that
+  # chech caching.
+  def clear_indexed_db(self):
+    shutil.copy(test_file('browser_reporting.js'), '.')
+    create_file('clear_indexed_db.html', '''
+      <script src="browser_reporting.js"></script>
+      <script>
+        // Clear the cache, so that the next test starts from a clean slate.
+        indexedDB.databases().then(dbs => {
+          Promise.all(dbs.map(db => {
+            return indexedDB.deleteDatabase(db.name);
+          })).then(() => {
+            reportResultToServer("clear");
+          });
+        });
+      </script>
+    ''')
+    self.run_browser('clear_indexed_db.html', '/report_result?clear')
+
   @parameterized({
     '0': (0,),
     '1mb': (1 * 1024 * 1024,),
     '100mb': (100 * 1024 * 1024,),
     '150mb': (150 * 1024 * 1024,),
   })
   def test_preload_caching(self, extra_size):
+    self.clear_indexed_db()
     self.set_setting('EXIT_RUNTIME')
     create_file('main.c', r'''
       #include <assert.h>
@@ -596,8 +616,9 @@ def test_preload_caching(self, extra_size):
           var packages = Object.keys(Module['preloadResults']);
           packages.forEach(function(package) {
             var fromCache = Module['preloadResults'][package]['fromCache'];
-            if (fromCache)
-              ++ cached;
+            if (fromCache) {
+              cached++;
+            }
           });
           return cached;
         }