make sure that wxProcess always have a valid PID set; add test unit for wxExecute,wxShell,wxProcess based on the code in the console sample; add some more infos/notes in wxProcess docs

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59784 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/test_test.dsp

@@ -289,6 +289,10 @@ SOURCE=.\events\evthandler.cpp
 # End Source File
 # Begin Source File
 
+SOURCE=.\exec\exec.cpp
+# End Source File
+# Begin Source File
+
 SOURCE=.\streams\ffilestream.cpp
 # End Source File
 # Begin Source File