Option --only-from-xml for dpl test framework

Option --only-from-xml for dpl test framework

[Issue#]       LINUXWRT-743
[Feature]      Reading testcase to be launched from TCT xml file
[Cause]        Need for automatic launching only TCT tests.
[Solution]     Additional option for test binary.
Additonal changes:
 - printing number of testcases at startup
 - listing option will take into account --regexp and --only-from-xml
[Verification] Build and run tests from change. Try this change with wrt-extra tests with commit "W3C testcases names adjust for TCT".
Install TCT on target:
 - tct-widget01-w3c
 - tct-widget02-w3c
 - tct-wgtapi01-w3c
 - tct-wgtapi02-w3c
 - tct-widgetpolicy-w3c
 - tct-widgetdigsig-w3c

All commands below should print requested list of tescases from xml file
and do not complain about wrong XML file (note that list option prints now only test
to run (after filtering them)):
 wrt-extra-tests-plugins-w3c --list --only-from-xml=/usr/share/tct-wgtapi02-w3c-tests/tests.xml
 wrt-extra-tests-plugins-w3c --list --only-from-xml=/usr/share/tct-wgtapi01-w3c-tests/tests.xml
 wrt-extra-tests-w3c --list --only-from-xml=/usr/share/tct-widget01-w3c-tests/tests.xml
 wrt-extra-tests-w3c --list --only-from-xml=/usr/share/tct-widget02-w3c-tests/tests.xml
 wrt-extra-tests-w3c-widget-policy --list --only-from-xml=/usr/share/tct-widgetpolicy-w3c-tests/tests.xml
 wrt-extra-tests-w3c-digsig --list --only-from-xml=/usr/share/tct-widgetdigsig-w3c-tests/tests.xml #no testcases anyway

Change-Id: I1b62a5d17dc351c1f763fb1d9a877e1300bc6a9c