ci: parametrize DB port for unit tests

Otherwise we can't run tests in parallel on the same host. Also the
container name has to be different depending on executor.

Resolves errors like:
```
docker: Error response from daemon: Conflict.
    The container name "/status-go-test-db" is already in use by container "123...".
    You have to remove (or rename) that container to be able to reuse that name.
```
Resolves: #4040

Signed-off-by: Jakub Sokołowski <jakub@status.im>

_assets/ci/Jenkinsfile.tests

@@ -25,7 +25,8 @@ pipeline {
 
   environment {
     TARGET   = 'tests'
-    DB_CONT  = 'status-go-test-db'
+    DB_CONT  = "status-go-test-db-${env.EXECUTOR_NUMBER.toInteger() + 1}"
+    DB_PORT  = "${5432 + env.EXECUTOR_NUMBER.toInteger()}"
     TMPDIR   = "${WORKSPACE_TMP}"
     GOPATH   = "${WORKSPACE_TMP}/go"
     GOCACHE  = "${WORKSPACE_TMP}/gocache"
@@ -69,12 +70,15 @@ pipeline {
     }
 
     stage('Unit Tests') {
+      environment {
+        TEST_POSTGRES_PORT = "${env.DB_PORT}"
+      }
       steps { script {
         db = docker.image('postgres:9.6-alpine').withRun([
           "--name=${DB_CONT}",
-          '--env=POSTGRES_HOST_AUTH_METHOD=trust',
-          '--publish=5432:5432',
-        ].join(' ')) { c ->
+          "--env=POSTGRES_HOST_AUTH_METHOD=trust",
+          "--publish=${DB_PORT}:${DB_PORT}",
+        ].join(' '), "-p ${DB_PORT}") { c ->
           nix.shell('make generate-handlers', pure: true)
           nix.shell('make test-unit V=1', pure: false)
         }

postgres/helpers.go

@@ -2,15 +2,28 @@ package postgres
 
 import (
 	"database/sql"
+	"fmt"
+	"os"
+
 	// Import postgres driver
 	_ "github.com/lib/pq"
 )
 
-const (
-	DefaultTestURI = "postgres://postgres@127.0.0.1:5432/postgres?sslmode=disable"
-	DropTableURI   = "postgres://postgres@127.0.0.1:5432/template1?sslmode=disable"
+var (
+	DefaultTestDBHost = GetEnvDefault("TEST_POSTGRES_HOST", "localhost")
+	DefaultTestDBPort = GetEnvDefault("TEST_POSTGRES_PORT", "5432")
+	DefaultTestURI    = fmt.Sprintf("postgres://postgres@%s:%s/postgres?sslmode=disable", DefaultTestDBHost, DefaultTestDBPort)
+	DropTableURI      = fmt.Sprintf("postgres://postgres@%s:%s/template1?sslmode=disable", DefaultTestDBHost, DefaultTestDBPort)
 )
 
+func GetEnvDefault(key, fallback string) string {
+	value := os.Getenv(key)
+	if len(value) == 0 {
+		return fallback
+	}
+	return value
+}
+
 func ResetDefaultTestPostgresDB() error {
 	db, err := sql.Open("postgres", DropTableURI)
 	if err != nil {