Diagnostics: set "isTestRunning" to false initially for all cards

- Cards get their "isTestRunning" value from diagnostics-app which
means the property never gets assigned a value when testing these
cards in isolation.
- False makes sense as a default value since all buttons are enabled
initially and it's consistent with the value defined in diagnostics-app.

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: Ia9bba381691abb5566c01f834c7592d214bc4517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616738
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842746}

chromeos/components/diagnostics_ui/resources/battery_status_card.js

@@ -81,6 +81,7 @@ Polymer({
     /** @type {boolean} */
     isTestRunning: {
       type: Boolean,
+      value: false,
       notify: true,
     },
   },

chromeos/components/diagnostics_ui/resources/cpu_card.js

@@ -67,6 +67,7 @@ Polymer({
     /** @type {boolean} */
     isTestRunning: {
       type: Boolean,
+      value: false,
       notify: true,
     },
 

chromeos/components/diagnostics_ui/resources/memory_card.js

@@ -61,6 +61,7 @@ Polymer({
     /** @type {boolean} */
     isTestRunning: {
       type: Boolean,
+      value: false,
       notify: true,
     }
   },