roachtest: run nightlies with assertions enabled

[DarrylWong]

In #111949 we briefly enabled roachtest nightlies to run with the crdb_test flag on. This enabled roachtests to be run with runtime assertions as well as metamorphic constants. However, due to the "metamorphic" nature of metamorphic constants, it was difficult to test all possible combinations/constants and tests frequently flaked. To reduce noise, we disabled both runtime assertions and metamorphic constants in #114618.

Runtime assertions however, seemed to be more stable. In terms of infra flakes, we expect they might contribute to longer run times, but other failures are likely actual assertions hit or issues with the assertion themselves. We should look into reenabling just runtime assertions on roachtest nightlies.

Runtime assertions and metamorphic constants are both enabled by default when building with --crdb_test. To disable metamorphic constants, COCKROACH_INTERNAL_DISABLE_METAMORPHIC_TESTING=true should be passed in when the cluster is started.

Related work/issues also includes:

• The roachprod/roachtest framework uses metamorphic testing/constants and runtime assertions interchangeably. Now that we are only running with runtime assertions, we should make a clearer distinction.
• The metamorphic constant framework unconditionally logs constants on init. Passing in COCKROACH_INTERNAL_DISABLE_METAMORPHIC_TESTING=true on cluster startup is sufficient to disable constants in the cluster, but it won't stop logging on every cockroach binary invocation, i.e. ./cockroach workload
• Since we suspect this might lead to longer run times, we should route timeouts to test-eng while we let this change stabalize.

Jira issue: CRDB-41227

[blathers-crl]

cc @cockroachdb/test-eng