Readme updates based on a fresher Ubuntu installation experience (#627)

README.md

@@ -25,7 +25,7 @@ Pageserver consists of:
 On Ubuntu or Debian this set of packages should be sufficient to build the code:
 ```text
 apt install build-essential libtool libreadline-dev zlib1g-dev flex bison libseccomp-dev \
-libssl-dev clang
+libssl-dev clang pkg-config libpq-dev
 ```
 
 [Rust] 1.52 or later is also required.
@@ -108,6 +108,13 @@ postgres=# insert into t values(2,2);
 INSERT 0 1
 ```
 
+6. If you want to run tests afterwards (see below), you have to stop pageserver and all postgres instances you have just started:
+```sh
+> ./target/debug/zenith pg stop migration_check
+> ./target/debug/zenith pg stop main
+> ./target/debug/zenith stop
+```
+
 ## Running tests
 
 ```sh

test_runner/fixtures/zenith_fixtures.py

@@ -71,7 +71,9 @@ def pytest_configure(config):
         # This is bad; we don't want any of those processes polluting the
         # result of the test.
         # NOTE this shows as an internal pytest error, there might be a better way
-        raise Exception('found interfering processes running')
+        raise Exception(
+            'Found interfering processes running. Stop all Zenith pageservers, nodes, WALs, as well as stand-alone Postgres.'
+        )
 
 
 def determine_scope(fixture_name: str, config: Any) -> str: