build: add PCHs for ndnsec and unit tests, fine-tune the existing ones

A full debug+tests build now takes 10% less time with gcc-9 and 21% less
with clang-10. Release builds (without tests) are only marginally faster
with gcc and 7% faster with clang.

Change-Id: I494778fe44cecc6209819a49f48a03f4d16c36af
diff --git a/tests/wscript b/tests/wscript
index d343b1a..d7401ca 100644
--- a/tests/wscript
+++ b/tests/wscript
@@ -1,13 +1,13 @@
 # -*- Mode: python; py-indent-offset: 4; indent-tabs-mode: nil; coding: utf-8; -*-
 
-top = '..'
+top = '../'
 
 def build(bld):
     # common objects that can be shared among all tests
     bld.objects(target='tests-common',
-                features='pch',
                 source=bld.path.ant_glob('*.cpp'),
-                headers=['../ndn-cxx/impl/common-pch.hpp', 'boost-test.hpp'],
+                features='pch',
+                headers='tests-pch.hpp',
                 use='ndn-cxx BOOST')
 
     bld.recurse('benchmarks')