Limit non-backend critical profiling tests to SQLite

Limit non-backend critical profiling tests to SQLite

issues with backend-specific profiling should be limited
to tests that are explcitly against resultset, compiler, etc.

MySQL in particular has an often varying callcount that isn't
worth running these tests against nor is it worth profiling
them for other backends like Oracle and SQL Server.

Also add the REQUIRE_SQLALCHEMY_CEXT flag to
the regen_callcounts.tox.ini script, which is part of some review
somewhere but is needed here to generate callcounts correctly.

Add a "warmup" phase for some of the ORM tests for join conditions
that have varying profile counts based on whether mappings have been
used already or not; profiling should always be against the
"warmed up" version of a function.

Change-Id: If483820235fa4cc4360cbd067a9b68d83512d587
(cherry picked from commit 386012597b9e7aa2a7f987930d00b892ed54121d)