Disable parallel testing on Github Actions CI

Tests were failing with:
Error: Process completed with exit code 143.

This appears to mean that Github Actions killed the runner.

See https://github.com/jackc/pgx/actions/runs/8216337993/job/22470808811
for an example.

It appears Github Actions kills runners based on resource usage. Running
tests one at a time reduces the resource usage and avoids the problem.

Or at least that's what I presume is happening. It sure is fun debugging
issues on cloud systems where you have limited visibility... :(

fixes #1934

.github/workflows/ci.yml

@@ -106,7 +106,8 @@ jobs:
           git diff --exit-code
 
       - name: Test
-        run: go test -v -race ./...
+        # parallel testing is disabled because somehow parallel testing causes Github Actions to kill the runner.
+        run: go test -parallel=1 -v -race ./...
         env:
           PGX_TEST_DATABASE: ${{ matrix.pgx-test-database }}
           PGX_TEST_UNIX_SOCKET_CONN_STRING: ${{ matrix.pgx-test-unix-socket-conn-string }}
@@ -149,6 +150,7 @@ jobs:
         shell: bash
 
       - name: Test
-        run: go test -v -race  ./...
+        # parallel testing is disabled because somehow parallel testing causes Github Actions to kill the runner.
+        run: go test -parallel=1 -v -race  ./...
         env:
           PGX_TEST_DATABASE: ${{ steps.postgres.outputs.connection-uri }}