cmake: testing disabled by default with MSVC

Previously testing was enabled only if GCC || Clang, and I though this was a
"proxy" for the likelihood of Perl being available. Apparently it was not just
that, since MSVC gives me a lot of errors when trying to build tests.

Let's fix them later, and disable for now.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8f83404..1f556f3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,7 +5,14 @@
 option(ENABLE_ZLIB_SUPPORT "Build mbed TLS with zlib library." OFF)
 
 option(ENABLE_PROGRAMS "Build mbed TLS programs." ON)
-option(ENABLE_TESTING "Build mbed TLS tests." ON)
+
+
+# the test suites currently have compile errors with MSVC
+if(MSVC)
+    option(ENABLE_TESTING "Build mbed TLS tests." OFF)
+else()
+    option(ENABLE_TESTING "Build mbed TLS tests." ON)
+endif()
 
 set(CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE}
     CACHE STRING "Choose the type of build: None Debug Release Coverage ASan ASanDbg MemSan MemSanDbg Check CheckFull"