MSTest C++ unit test dll not working - c++

Question

C# Unit Test - Runs ok
I have read the MSTest unit test (utterly useless )documentation on the microsoft website here: utterly-useless-ms-doc-link. Then I read a whole load of other more useful links (e.g. on SO) and finally I am able to run C# unit tests no problems, like this:
mstest /testcontainer:UnitTest.dll /detail:errormessage
C++ Unit Test - Fails
As far as I can tell this should be the same for the C++ unit tests, so I ran a similar command (specific command details at the end) for C++, but then I get the error:
UnitTest_TrackManager.dll Unable to load the test container
'UnitTest_TrackManager.dll' or one of its dependencies. If you build
your test project assembly as a 64 bit assembly, it cannot be loaded.
When you build your test project assembly, select "Any CPU" for the
platform. To run your tests in 64 bit mode on a 64 bit processor, you
must change your test settings in the Hosts tab to run your tests in a
32 bit process. Error details: Could not load file or assembly
'file:///D:\MiddlewareTest01\x64\Debug\UnitTest_TrackManager.dll'
or one of its dependencies. The module was expected to contain an
assembly manifest.
32-bit or 64-bit DLL?
So, fair enough, I have built the code for 64 bit and MStest is 32bit only. I used dumpbin /HEADERS fname.dll to check that this is a 64-bit file and it is:
File Type: DLL
FILE HEADER VALUES
8664 machine (x64)
D number of sections
5829E628 time date stamp Mon Nov 14 16:28:24 2016
0 file pointer to symbol table
0 number of symbols
F0 size of optional header
2022 characteristics
Executable
Application can handle large (>2GB) addresses
DLL
So, then I recompie for 32-bit and run the test again, but I get the same error. So I check that the file output (the DLL) is 32-bit and it does appear to be, here is the dumpbin for that:
File Type: DLL
FILE HEADER VALUES
14C machine (x86)
C number of sections
5829E792 time date stamp Mon Nov 14 16:34:26 2016
0 file pointer to symbol table
0 number of symbols
E0 size of optional header
2102 characteristics
Executable
32 bit word machine
DLL
C++ Unit Test Works in the IDE
Using MSVS2015 IDE, I can compile and run my code/unit tests and they all work (when I use the x86 build) - and they all pass. If I select x64 build then it complains.
Specific Command That Fails
Here is the exact command line I am useing:
mstest /testcontainer:UnitTest_TrackManager.dll /detail:errormessage
It is run from the same location as the dll lives which is here:
D:\<somepath>\MiddlewareTest01\Debug\UnitTest_TrackManager.dll
Where <somepath> is just there to hide my PC/user details and MiddlewareTest01 is the solution folder.
As I say, I can get this to work for C# projects, but for C++ it does not seem to work in the same way.
Update
Using dependency-walker it tells me:
MICROSOFT.VISUALSTUDIO.TESTTOOLS.CPPUNITTESTFRAMEWORK.DLL was not found. Could this be an issue?