Support multiple books in the GUI tests

This adds the ability to use multiple books for the GUI tests. This is
helpful since some tests need special configuration, and sharing the
same book can make it difficult or impossible to test different
configurations. It also makes it difficult to make changes to the
test_book since it can affect other tests.

This works by placing the books in the tests/gui/books directory. The
test runner will automatically build all the books. The gui tests can
then just access the DOC_PATH with the name of the book.

Books are now saved in a temp directory to make it easier to use the
DOC_PATH variable, instead of being tests/gui/books/book_name/book which
is a little awkward.

Following commits will restructure the existing book. This is just a
mechanical move.

Author: ehuss

.gitignore

@@ -8,7 +8,7 @@ guide/book
 
 .vscode
 tests/dummy_book/book/
-test_book/book/
+tests/gui/books/*/book/
 tests/testsuite/*/*/book/
 
 # Ignore Jetbrains specific files.