[tests] Ensure that the correct msbuild.dll.config gets picked up

.. by setting `MSBUILD_EXE_PATH` to the corresponding `MSBuild.dll`.

tests/Makefile

@@ -41,6 +41,7 @@ EXEC_UNIT_SERVER=XCODE_DEVELOPER_ROOT=$(XCODE_DEVELOPER_ROOT) MONOTOUCH_ROOT=$(I
 
 export MD_APPLE_SDK_ROOT=$(abspath $(XCODE_DEVELOPER_ROOT)/../..)
 export MD_MTOUCH_SDK_ROOT=$(IOS_DESTDIR)/$(MONOTOUCH_PREFIX)
+export MSBUILD_EXE_PATH=$(MONO_PREFIX)/lib/mono/msbuild/15.0/bin/MSBuild.dll
 export TargetFrameworkFallbackSearchPaths=$(IOS_DESTDIR)/Library/Frameworks/Mono.framework/External/xbuild-frameworks
 export MSBuildExtensionsPathFallbackPathsOverride=$(IOS_DESTDIR)/Library/Frameworks/Mono.framework/External/xbuild
 export XAMMAC_FRAMEWORK_PATH=$(MAC_DESTDIR)/Library/Frameworks/Xamarin.Mac.framework/Versions/Current