The new version of jtreg allows to use ${test.main.class} to refer to the test main class on the `@run` command line.

This is already used in https://github.com/openjdk/jdk/blob/master/test/jdk/java/net/httpclient/http3/H3RequestRejectedTest.java#L68

Other httpclient tests should be refactored to make use of this new facility.
Since new tests are often created by copying old tests, this would avoid mistakes where the new test file in fact runs the test class it was copied from.

There will be some exceptions, in particular with tests under jdk/java/net/httpclient/whitebox which have a Driver file which only contains the jtreg test comment.