Some fixes for the testsuite

With these fixes, unit tests pass with MacOS
(set DYLD_LIBRARY_PATH!)

1.) MacOS takes popen vs. pclose seriously
By using fopen we basically spoil the system and other popen
won't work.

2.) For system integrity, MacOS does not propagate DYLD_LIBRARY_PATH
to child processes such as sh. This has to be done explictly.

3.) Search the klayout binary in the right path (klayout.app/...)

4.) Reset KLAYOUT_HOME for less intervention by installed macros

testdata/klayout_main/main.rb

@@ -30,8 +30,25 @@ load("test_prologue.rb")
 
 class KLayoutMain_TestClass < TestBase
 
+  def setup
+    @klayout_home_name = "KLAYOUT_HOME"
+    @klayout_home = ENV[@klayout_home_name]
+    # setting "KLAYOUT_HOME" to empty means we don't search any place
+    # for macros
+    ENV[@klayout_home_name] = ""
+  end
+
+  def teardown
+    ENV[@klayout_home_name] = @klayout_home
+  end
+
   def klayout_bin
-    File.join(RBA::Application::instance.inst_path, "klayout")
+    # special location for MacOS
+    file = File.join(RBA::Application::instance.inst_path, "klayout.app", "Contents", "MacOS", "klayout")
+    if !File.exists?(file)
+      file = File.join(RBA::Application::instance.inst_path, "klayout")
+    end
+    return file
   end
 
   def test_1