Introduce a new, per-test-program TEST_METADATA.<program> variable that
contains a list of key/value paris describing metadata properties for
that test program. These properties are later written into the
auto-generated Kyuafile when using the KYUAFILE=auto functionality.
This is to avoid having to supply hand-crafted Kyuafiles when the needs
for metadata overrides are trivial.
While doing this, and because I am documenting TEST_METADATA, take the
chance to document the TEST_INTERFACE setting as well.
MFC after: 5 days
8c465f5880