Add support for trigger_sources

In addition to the previously-implemented support for "target_sources" properties for test cases, add support for the "trigger_sources".

The new property can contain either a regular expression, or a list thereof, matching the source files the test case is supposed to be covering, but not enough to call either of them "targeted".

A test receives the sum of "trigger_sources" properties from the case and its parents, similar to "target_sources". Now, if a test has both "trigger_sources" and "target_sources" missing, only then it is considered to be covering something outside the source tree, and is included regardless of whether or not (or which) source files were specified to a kpet command.

"Targeted" tests are also considered "triggered", but not vice versa.

Add support for extra condition for including "high-cost" cases with the "--high-cost" option: "triggered". Default "kpet run" to the new value.

This value would include "high-cost" cases when either "target_sources" or "trigger_sources" match the supplied source files. The previously-supported "targeted" value would include them only if the sources matched the "target_sources".