Document DejaGnu dependency and manual tests

The Texinfo install guide had a separate subsection about the DejaGnu
tests which did not get converted to the RST source.

In the testing section, also link to the wiki page on manual testing.

https://github.com/krb5/krb5/commit/fd136728de5b603974e3ec21bc5856406c4e814d
Author: Ben Kaduk <kaduk@mit.edu>
Commit: fd136728de5b603974e3ec21bc5856406c4e814d
Branch: master
 doc/rst_source/krb_build/doing_build.rst |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)