testament: joinable is now an explicit concept of a test spec

tests/system/t7894.nim

@@ -1,6 +1,7 @@
 discard """
 disabled: "travis"
 disabled: "appveyor"
+joinable: false
 """
 
 # CI integration servers are out of memory for this test

tests/system/talloc2.nim

@@ -1,5 +1,6 @@
 discard """
 disabled: "windows"
+joinable: false
 """
 
 # appveyor is "out of memory"

tests/system/tparams.nim

@@ -1,3 +1,8 @@
+discard """
+joinable: false
+"""
+
+# not joinable because it executes itself with parameters
 import os
 import osproc
 import parseopt2

tests/system/trealloc.nim

@@ -1,5 +1,6 @@
 discard """
   output: '''success'''
+  joinable: false
 """
 
 # bug #4818