tests: Replace 'make extra-tests' with individual tests.

'make extra-tests' was a monolithic set of tests that did all sorts of
things: valgrind, tests over local guests, tests with upstream qemu,
tests with upstream libvirt, tests with the appliance attach method.

This made it hard to perform individual tests, eg. just valgrind
testing.  It was also hard to maintain because the tests were not
located in the same directories as the programs and sometimes
duplicated tests that were run elsewhere.

This commit splits up 'make extra-tests' into 5 separate targets:

 make check-valgrind              # run a subset of tests under valgrind
 make check-valgrind-local-guests # test under valgrind with local guests
 make check-with-appliance        # test with attach-method == appliance
 make check-with-upstream-qemu    # test with an alternate/upstream qemu
 make check-with-upstream-libvirt # test with an alternate/upstream libvirt

(You can also still run 'make extra-tests' which is now simply
a rule that runs the above 5 targets in order).

This replaces everything that was in the tests/extra directory,
so that has now gone.

README

@@ -164,13 +164,19 @@ Finally run the tests:
 
   make check
 
-There are some extra tests, but these require that you have some
-libvirt guests installed, that these guests' disks are accessible by
-the current user, and these tests may fail for other reasons which are
-not necessarily because of real problems.  If you want to run these
-extra tests do:
+Also:
 
-  make extra-tests
+ make check-valgrind
+
+runs a subset of the test suite under valgrind (requires valgrind to
+be installed obviously).
+
+ make extra-tests
+
+runs check-valgrind + even more tests, but these require that you have
+some libvirt guests installed, that these guests' disks are accessible
+by the current user, and these tests may fail for other reasons which
+are not necessarily because of real problems.
 
 If everything works, you can install the library and tools by running
 this command as root: