adding some provisions for writing lower-level unit tests targeting specific sub-systems of the compiler

see sigmatch as an example.
tests are compiled only when the compiler is compiled with -d:selftest
to execute them, just run the resulting binary without arguments

compiler/testability.nim

@@ -0,0 +1,5 @@
+template tests*(body: stmt) {.immediate.} =
+  when defined(selftest):
+    when not defined(unittest): import unittest
+    body
+