delay for WillDelayFor() duration, before returning error set by WillReturnError()

If both WillDelayFor() and WillReturnError() are called, there should
be a delay before the error is returned.  Applied to Begin, Exec,
Query, and Prepare.  Also, the context versions of same.

And a couple of unit tests.

sqlmock_test.go

@@ -2,6 +2,7 @@ package sqlmock
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"strconv"
 	"sync"
@@ -1063,3 +1064,52 @@ func TestPreparedStatementCloseExpectation(t *testing.T) {
 		t.Errorf("there were unfulfilled expectations: %s", err)
 	}
 }
+
+func TestExecExpectationErrorDelay(t *testing.T) {
+	t.Parallel()
+	db, mock, err := New()
+	if err != nil {
+		t.Errorf("an error '%s' was not expected when opening a stub database connection", err)
+	}
+	defer db.Close()
+
+	// test that return of error is delayed
+	var delay time.Duration
+	delay = 100 * time.Millisecond
+	mock.ExpectExec("^INSERT INTO articles").
+		WillReturnError(errors.New("slow fail")).
+		WillDelayFor(delay)
+
+	start := time.Now()
+	res, err := db.Exec("INSERT INTO articles (title) VALUES (?)", "hello")
+	stop := time.Now()
+
+	if res != nil {
+		t.Errorf("result was not expected, was expecting nil")
+	}
+
+	if err == nil {
+		t.Errorf("error was expected, was not expecting nil")
+	}
+
+	if err.Error() != "slow fail" {
+		t.Errorf("error '%s' was not expected, was expecting '%s'", err.Error(), "slow fail")
+	}
+
+	elapsed := stop.Sub(start)
+	if elapsed < delay {
+		t.Errorf("expecting a delay of %v before error, actual delay was %v", delay, elapsed)
+	}
+
+	// also test that return of error is not delayed
+	mock.ExpectExec("^INSERT INTO articles").WillReturnError(errors.New("fast fail"))
+
+	start = time.Now()
+	db.Exec("INSERT INTO articles (title) VALUES (?)", "hello")
+	stop = time.Now()
+
+	elapsed = stop.Sub(start)
+	if elapsed > delay {
+		t.Errorf("expecting a delay of less than %v before error, actual delay was %v", delay, elapsed)
+	}
+}