Due to issue 399499, the output in the build step may not contain the full output of the test. To see the full output, click the stdio link for the steps build step and search for test_installer.py. The most common failure will be a failure to launch Chrome, which generally indicates that chrome.exe is crashing at startup.
First build the mini_installer target. The minimal command to run the test is:
python chrome\test\mini_installer\test_installer.py --config chrome\test\mini_installer\config\config.config [test_name]
Specify individual test names as, for example: __main__/InstallerTest/ChromeUserLevel.
Run it with --help for more options. Note that you should use the Python that comes with depot_tools as other versions might not work.
Contact grt at chromium dot org.