change file extension .qbk -> .quickbook.

make quickbook-testing.jam independent of quickbook.jam toolset.
compile text_diff and quickbook only once when there are multiple tests.


[SVN r30876]

tools/quickbook

@@ -1 +1 @@
-Subproject commit 83ac3b00b32a61fdf3195cf2af0de9d3104415e8
+Subproject commit 9350d67090074b116c7ed8f2e8aa07abe140b74d