mark fail, mustExec and mustQuery as test helpers

[shogo82148]

Description

If an error occurs, fail, mustExec, and mustQuery report the line in the fail method where the error occurred.
For example, here is a broken test:

diff --git a/driver_test.go b/driver_test.go
index dd3d731..f7fab0d 100644
--- a/driver_test.go
+++ b/driver_test.go
@@ -198,6 +198,15 @@ func maybeSkip(t *testing.T, err error, skipErrno uint16) {
 	}
 }
 
+func TestMustFail(t *testing.T) {
+	runTests(t, dsn, func(dbt *DBTest) {
+		rows := dbt.mustQuery("WRONG QUERY")
+		defer rows.Close()
+		for rows.Next() {
+		}
+	})
+}
+
 func TestEmptyQuery(t *testing.T) {
 	runTests(t, dsn, func(dbt *DBTest) {
 		// just a comment, no query

The result of go test is:

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestMustFail$ github.com/go-sql-driver/mysql -v -count=1 -race

=== RUN   TestMustFail
=== RUN   TestMustFail/default
    /Users/shogo/src/github.com/go-sql-driver/mysql/driver_test.go:171: error on query WRONG QUERY: Error 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WRONG QUERY' at line 1
=== RUN   TestMustFail/interpolateParams
    /Users/shogo/src/github.com/go-sql-driver/mysql/driver_test.go:171: error on query WRONG QUERY: Error 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WRONG QUERY' at line 1
--- FAIL: TestMustFail (0.03s)
    --- FAIL: TestMustFail/default (0.00s)
    --- FAIL: TestMustFail/interpolateParams (0.00s)
FAIL
FAIL	github.com/go-sql-driver/mysql	0.193s

go test reports driver_test.go:171. driver_test.go:171 is here:

func (dbt *DBTest) fail(method, query string, err error) {
	dbt.Helper()
	if len(query) > 300 {
		query = "[query too large to print]"
	}
	dbt.Fatalf("error on %s %s: %s", method, query, err.Error()) // <- go test reports this position.
}

By adding dbt.Helper(), the result of go test becomes:

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestMustFail$ github.com/go-sql-driver/mysql -v -count=1 -race

=== RUN   TestMustFail
=== RUN   TestMustFail/default
    /Users/shogo/src/github.com/go-sql-driver/mysql/driver_test.go:206: error on query WRONG QUERY: Error 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WRONG QUERY' at line 1
=== RUN   TestMustFail/interpolateParams
    /Users/shogo/src/github.com/go-sql-driver/mysql/driver_test.go:206: error on query WRONG QUERY: Error 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WRONG QUERY' at line 1
--- FAIL: TestMustFail (0.03s)
    --- FAIL: TestMustFail/default (0.00s)
    --- FAIL: TestMustFail/interpolateParams (0.01s)
FAIL
FAIL	github.com/go-sql-driver/mysql	0.204s

func TestMustFail(t *testing.T) {
	runTests(t, dsn, func(dbt *DBTest) {
		rows := dbt.mustQuery("WRONG QUERY") // <- go test reports this position.
		defer rows.Close()
		for rows.Next() {
		}
	})
}

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[coveralls]

coverage: 82.713%. remained the same when pulling 870fecb on mark-as-helper into 3798012 on master.