TemplateBaseTest: Minor style/header guard fixes
TestFixture: Small style cleanup
TestFixture: Add ExecuteCode() version for multiple files
Pull base class DataBasedTest out of TemplateBasedTest
DataBasedTestParser: Add support for multiple file tests

* The directive "#multipleJamfiles" in the code section (which is
  ignored save for directives in this case) specifies that the test
  cases are multi-file tests. The input section is supposed to contain
  a "#!file" directive with the file name as an argument, indicating the
  start of the content for that file.
* Add MultipleFilesDataBasedTest class for the multiple files tests.
TestFixture::ExecutableExecuter -> CodeExecuter

It no longer only executes executables, so the new name is more
accurate.
Add code/Include, some tests for "include"