webtest-*.ebuild: Add lxml to DEPEND[test]

webtest-*.ebuild: Add lxml to DEPEND[test]

Avoid:

  File "/var/tmp/portage/dev-python/webtest-1.4.3-r1/work/WebTest-1.4.3/docs/index.txt", line 320, in index.txt
  Failed example:
      res.lxml
  Exception raised:
      Traceback (most recent call last):
        File "/var/tmp/portage/dev-python/webtest-1.4.3-r1/work/WebTest-1.4.3/webtest/app.py", line 568, in lxml
          from lxml import etree
      ImportError: No module named 'lxml'

      During handling of the above exception, another exception occurred:

      Traceback (most recent call last):
        File "/usr/lib64/python3.3/doctest.py", line 1313, in __run
          compileflags, 1), test.globs)
        File "<doctest index.txt[22]>", line 1, in <module>
          res.lxml
        File "/var/tmp/portage/dev-python/webtest-1.4.3-r1/work/WebTest-1.4.3/webtest/app.py", line 571, in lxml
          "You must have lxml installed to use response.lxml")
      ImportError: You must have lxml installed to use response.lxml

and:

  File "/var/tmp/portage/dev-python/webtest-2.0.11/work/WebTest-2.0.11-python3_3/docs/testresponse.txt", line 124, in testresponse.txt
  Failed example:
      res.lxml
  Exception raised:
      Traceback (most recent call last):
        File "/var/tmp/portage/dev-python/webtest-2.0.11/work/WebTest-2.0.11-python3_3/webtest/response.py", line 477, in lxml
          from lxml import etree
      ImportError: No module named 'lxml'

      During handling of the above exception, another exception occurred:

      Traceback (most recent call last):
        File "/usr/lib64/python3.3/doctest.py", line 1313, in __run
          compileflags, 1), test.globs)
        File "<doctest testresponse.txt[8]>", line 1, in <module>
          res.lxml
        File "/var/tmp/portage/dev-python/webtest-2.0.11/work/WebTest-2.0.11-python3_3/webtest/response.py", line 480, in lxml
          "You must have lxml installed to use response.lxml")
      ImportError: You must have lxml installed to use response.lxml