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/nimrod.nim

@@ -76,5 +76,7 @@ when compileOption("gc", "v2") or compileOption("gc", "refc"):
   # the new correct mark&sweet collector is too slow :-/
   GC_disableMarkAndSweep()
 condsyms.InitDefines()
-HandleCmdLine()
-quit(int8(msgs.gErrorCounter > 0))
+
+when not defined(selftest):
+  HandleCmdLine()
+  quit(int8(msgs.gErrorCounter > 0))