These are convenience targets for running specific tests as a developer.
They are more useful that invoking run-tests directly as they take account
of the VARIANT= specified on the make command-line.
For instance, you can run all tests matching the regular expression "int"
with `make VARIANT=... test//int`.
The new targets are all documented in README.md.
Signed-off-by: Jeff Epler <jepler@gmail.com>