Rename the testbuild/torbrowser-testbuild targets

For release, alpha and nightly we use the same target name in projects/release/config and other components.

For testbuild however, we use the testbuild target in projects/release/config, which is using the torbrowser-testbuild target (defined in rbm.conf) for building other components, which can be confusing. There is also a testbuild target in rbm.conf, which is used by torbrowser-testbuild.

I think we could fix that by renaming the torbrowser-testbuild target we have in rbm.conf to testbuild, and the current testbuild target to testbuild-common.

This will however require that people who use a custom rbm.local.conf update it.