Add log adapters for testing and log15

Make LogLevel a type for Stringer interface.

log/testingadapter/adapter.go

@@ -0,0 +1,25 @@
+// Package testingadapter provides a logger that writes to a test or benchmark
+// log.
+package testingadapter
+
+import (
+	"github.com/jackc/pgx"
+)
+
+// TestingLogger interface defines the subset of testing.TB methods used by this
+// adapter.
+type TestingLogger interface {
+	Log(args ...interface{})
+}
+
+type Logger struct {
+	l TestingLogger
+}
+
+func NewLogger(l TestingLogger) *Logger {
+	return &Logger{l: l}
+}
+
+func (l *Logger) Log(level pgx.LogLevel, msg string, ctx ...interface{}) {
+	l.l.Log(level, msg, ctx)
+}