Add tests for the bug IOTSSL-619.

The main goal with these tests is to test the bug in question and
they are not meant to test the entire PKCS#1 v1.5 behaviour. To
achieve full test coverage, further test cases are needed.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 1cca818..dfef1ef 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -82,6 +82,7 @@
 add_test_suite(memory_buffer_alloc)
 add_test_suite(mpi)
 add_test_suite(pem)
+add_test_suite(pkcs1_v15)
 add_test_suite(pkcs1_v21)
 add_test_suite(pkcs5)
 add_test_suite(pk)