Test are being shown under "External" category in Test Explorer when Grouped By Project

This is due the pdb file is not included in the compilation. If you're compiling in a solution configuration other than debug you must ensure that pdb is included.

To achieve that you must open the advanced build setting:

• Right click on project and select properties
• Click on build tab
• Check that your configuration is selected in the "Configuration" dropdown
• Click on advanced button.

In the new window you must select "pdb-only" in the debug info dropdown.

This was happening for me but only when the combined length of the namespace, class, and test method exceeded 254 chars. (MS-Test, Visual Studio 2013 12.0.40629.00 Update 5)

namespace MyTests.HaveLong.Complicated.NamespaceAsWell.AsMuchAs.EightyFive.Chars.IfThatMatters
{
    [TestClass]
    public class A_Long_ClassName_MayAlso_Contribute_ToThe_Issue_ThisOneIs_EightyFive_Characters_Long
    {
        [TestMethod]
        public async Task This_IsAMsTest_TestMethod_WithAnEightyFive_CharacterName_WhichWillShow_UnderExternal()
        {
        }
        [TestMethod]
        public async Task This_IsAMsTest_TestMethod_WithAnEightyFour_CharacterName_WhichWontShowUnderExternal()
        {
        }
    }
}