Useful Links:
How do you execute specific test cases?
Execute ant test and set the gwt.junit.testcase system property to an ant-style pattern.
example: ant test -Dgwt.junit.testcase.includes=com/google/gwt/core/CoreSuite.class
How do you execute specific test cases that wait for a debug connection to be established?
Execute ant test and set the junit.platform.args system property to allow remote debugger connections.
This works on windows but not linux... example: ant test -Djunit.platform.args="-agentlib:jdwp=transport=dt_socket,server=y,timeout=10000,address=8000"
See JPDA Connection and Invocation for a description of -agentlib and associated options.
How do you implement a benckmark?
The GWT documentation for JUnit integration and Benchmarking is the first place to start. The javadoc for the Benchmark class that has all the documentation for understanding, implementing and executing benchmarks. Refer to the GWT trunk source for the Benchmark class for the most up to date information.