Add a dedicated Mainline Test Mapping group for Cellbroadcast

Test of *all* Mainline modules are currently configured in a single
`mainline-presubmit` Test Mapping group. This requires that users indicate the
module to install in every entry and is quite tedious.

The above approach also adds overhead due to installing, checking for, and
uninstalling Mainline modules between test module executions. This eats up
precious presubmit time and gets runtimes close to violating the SLO.

This change moves all Cellbroadcast Mainline module tests into a dedicated
Test Mapping group that installs the Mainline module once before executing all
test modules. This also simplifies the configuration syntax by no longer
requiring brackets that indicate the, now implicit, Mainline module.

Bug: 328102821
Test: presubmit checks
Change-Id: I1a6a89435ccd37ae12931bb9870dc24590c44887