ctest_update: Add support for Perforce p4 client

Teach the ctest_update implementation to use the p4 command-line
client to perform updates and extract the list of changes.

Add a CTest.UpdateP4 test like those that exist already for the other
version control tools.  Make the test available when p4 and the p4d
server are found.  During the test launch p4d in the background to
serve a repository from the test directory.  Then direct the client
toward this server for the duration of the test.
Merge topic 'ctest-p4' into next

c63ecee ctest_update: Add support for Perforce p4 client