Use compute_ctl to manage Postgres in tests. (#3886)

This adds test coverage for 'compute_ctl', as it is now used by all
the python tests.
    
There are a few differences in how 'compute_ctl' is called in the
tests, compared to the real web console:
    
- In the tests, the postgresql.conf file is included as one large
  string in the spec file, and it is written out as it is to the data
  directory.  I added a new field for that to the spec file. The real
  web console, however, sets all the necessary settings in the
  'settings' field, and 'compute_ctl' creates the postgresql.conf from
  those settings.

- In the tests, the information needed to connect to the storage, i.e.
  tenant_id, timeline_id, connection strings to pageserver and
  safekeepers, are now passed as new fields in the spec file. The real
  web console includes them as the GUCs in the 'settings' field. (Both
  of these are different from what the test control plane used to do:
  It used to write the GUCs directly in the postgresql.conf file). The
  plan is to change the control plane to use the new method, and
  remove the old method, but for now, support both.

Some tests that were sensitive to the amount of WAL generated needed
small changes, to accommodate that compute_ctl runs the background
health monitor which makes a few small updates. Also some tests shut
down the pageserver, and now that the background health check can run
some queries while the pageserver is down, that can produce a few
extra errors in the logs, which needed to be allowlisted.

Other changes:
- remove obsolete comments about PostgresNode;
- create standby.signal file for Static compute node;
- log output of `compute_ctl` and `postgres` is merged into
`endpoints/compute.log`.

---------

Co-authored-by: Anastasia Lubennikova <anastasia@neon.tech>

compute_tools/src/pg_helpers.rs

@@ -23,7 +23,7 @@ fn escape_literal(s: &str) -> String {
 
 /// Escape a string so that it can be used in postgresql.conf.
 /// Same as escape_literal, currently.
-fn escape_conf_value(s: &str) -> String {
+pub fn escape_conf_value(s: &str) -> String {
     s.replace('\'', "''").replace('\\', "\\\\")
 }