README: make it a bit clearer how to get regression tests running (#4885)

## Problem

When setting up for the first time I hit a couple of nits running tests:
- It wasn't obvious that `openssl` and `poetry` were needed (poetry is
mentioned kind of obliquely via "dependency installation notes" rather
than being in the list of rpm/deb packages to install.
- It wasn't obvious how to get the tests to run for just particular
parameters (e.g. just release mode)

## Summary of changes

Add openssl and poetry to the package lists.

Add an example of how to run pytest for just a particular build type and
postgres version.

README.md

@@ -29,13 +29,13 @@ See developer documentation in [SUMMARY.md](/docs/SUMMARY.md) for more informati
 ```bash
 apt install build-essential libtool libreadline-dev zlib1g-dev flex bison libseccomp-dev \
 libssl-dev clang pkg-config libpq-dev cmake postgresql-client protobuf-compiler \
-libcurl4-openssl-dev
+libcurl4-openssl-dev openssl python-poetry
 ```
 * On Fedora, these packages are needed:
 ```bash
 dnf install flex bison readline-devel zlib-devel openssl-devel \
   libseccomp-devel perl clang cmake postgresql postgresql-contrib protobuf-compiler \
-  protobuf-devel libcurl-devel
+  protobuf-devel libcurl-devel openssl poetry
 ```
 * On Arch based systems, these packages are needed:
 ```bash
@@ -235,6 +235,13 @@ CARGO_BUILD_FLAGS="--features=testing" make
 ./scripts/pytest
 ```
 
+By default, this runs both debug and release modes, and all supported postgres versions. When
+testing locally, it is convenient to run just run one set of permutations, like this:
+
+```sh
+DEFAULT_PG_VERSION=15 BUILD_TYPE=release ./scripts/pytest
+```
+
 ## Documentation
 
 [docs](/docs) Contains a top-level overview of all available markdown documentation.