[Dcmlib] TestVR is failing on VS6

[Mathieu Malaterre]

> Now, it contains tests that really test the VR
> Thanks to me

Thank you Benoit, you are my hero :-)

Anyway I found the bug. This is yet another feature of VS6. You CANNOT 
have iterator in two different context (shared lib and executable). What 
happen in this case is that .end() is a static and has a different value 
in the gdcm.dll compare to the gdcmTests.exe. Therefore testing if an 
iterator is passed the end has no meaning since the value of .edn is 
different from one dll to the other.

Solution: Move the code in the cxx, so that IsValid is actually only in 
one place.

Mathieu