To find test reports, follow the instructions for the build server you're using.

TeamCity

To display a list of test results including failures, click the Tests tab:

For more details about a specific test, click the  arrow.

TFS

The file containing test reports is saved in the drop folder you specified when setting up the build definition. To see this in Visual Studio, once your build is complete, click Open Drop Folder:

The test report is saved as <BuildDefinitionName>reports.junit.xml. 

SQL CI command line

When you're using the test step command line options, the output folder for test results is the current working directory unless you include this option in the test step:

/outputFolder="<filelocation>" 

For example:

Test /package=WidgetShop\Database\Output\WidgetShop.1.0.nupkg /temporaryDatabaseServer=(localdb)\v11.0 /outputFolder="c:\WidgetShoptestresults"