tests: add unit tests to check the activation of PBTS

There is a test unit `pbts_test.go` only dedicated to PBTS. Those tests, however, assume that PBTS is enabled by default, which is true in unit tests (see https://github.com/cometbft/cometbft/issues/2324). Since we need to support the legacy BFT Time algorithm, we should validate the transition from BFT Time to PBTS.