ENH: Add ExtensionBuildSystem tests

This commit adds four tests (listed below). At the time of this
commit, tests have been confirmed to pass on Linux, MacOSX and
Windows.

For each tests, three extensions (each bundling one scripted module) are
generated: TestExtA, TestExtB and TestExtC.

The generated sources are programmatically edited adding TestExtA and
TestExtB as dependencies of TestExtC.

At the end of each tests, the following are checked:

• the CMakeCache.txt
• the list of requests made to CDash and Midas

List of tests:

• py_cmake_slicer_extensions_index_build_without_upload:

  Build the extension index with option Slicer_UPLOAD_EXTENSIONS set to 0

• py_cmake_slicer_extensions_index_build_with_upload

  Build the extension index with option Slicer_UPLOAD_EXTENSIONS set to 1

• py_cmake_slicer_extensions_index_build_with_upload_using_ctest

  Build the extension index with option Slicer_UPLOAD_EXTENSIONS set to 1
  using ctest. This is similar to the build of extensions done using
  dashboard script on the factory.

• py_cmake_slicer_extensions_index_build_without_upload_using_ctest

  Build the extension index with option Slicer_UPLOAD_EXTENSIONS set to 0
  using ctest.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25296 3bd1e089-480b-0410-8dfb-8563597acbee