Don't raise an error for valid microvias

When the layers are adjacent, the microvia is _probably_ valid.  This is
not the case when trying to run a microvia through the core material but
without stackup information, we cannot be certain.

Note that microvias are not required in current manufacturing techniques
to be blind.

KSC Issue '400'

pcbnew/drc_clearance_test_functions.cpp

@@ -296,6 +296,8 @@ bool DRC::doTrackDrc( TRACK* aRefSeg, TRACK* aStart, bool aTestPads, bool aTestZ
                 err = false;
             else if( layer1 == F_Cu  &&  layer2 == In1_Cu  )
                 err = false;
+            else if( static_cast<int>( layer2 ) - static_cast<int>( layer1 ) == 1 )
+                err = false;
 
             if( err )
             {