tests: select testdb and use MySQL 8.0 by default

Without the `USE...` in `TestExecuteMultiple` the test panics as the
result in `conn_test.go:108` is nil.

```
$ go test

----------------------------------------------------------------------
PANIC: conn_test.go:92: connTestSuite.TestExecuteMultiple

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x438592)

/usr/lib/golang/src/runtime/panic.go:884
  in gopanic
/usr/lib/golang/src/runtime/panic.go:260
  in panicmem
/usr/lib/golang/src/runtime/signal_unix.go:837
  in sigpanic
conn_test.go:108
  in connTestSuite.TestExecuteMultiple.func1
conn.go:273
  in Conn.ExecuteMultiple
conn_test.go:101
  in connTestSuite.TestExecuteMultiple
/usr/lib/golang/src/reflect/value.go:370
  in Value.Call
/home/dvaneeden/go/pkg/mod/github.com/pingcap/[email protected]/check.go:836
  in suiteRunner.forkTest.func1
/home/dvaneeden/go/pkg/mod/github.com/pingcap/[email protected]/check.go:730
  in suiteRunner.forkCall.func1
/usr/lib/golang/src/runtime/asm_amd64.s:1598
  in goexit

----------------------------------------------------------------------
FAIL: conn_test.go:132: connTestSuite.TestExecuteSelectStreaming

conn_test.go:170:
    c.Assert(err, IsNil)
... value *mysql.MyError = &mysql.MyError{Code:0x416, Message:"No database selected", State:"3D000"} ("ERROR 1046 (3D000): No database selected")

----------------------------------------------------------------------
FAIL: conn_test.go:86: connTestSuite.TestFieldList

conn_test.go:88:
    c.Assert(err, IsNil)
... value *mysql.MyError = &mysql.MyError{Code:0x416, Message:"No database selected", State:"3D000"} ("ERROR 1046 (3D000): No database selected")

----------------------------------------------------------------------
FAIL: conn_test.go:38: connTestSuite.TearDownSuite

conn_test.go:43:
    s.testExecute_DropTable(c)
conn_test.go:83:
    c.Assert(err, IsNil)
... value *mysql.MyError = &mysql.MyError{Code:0x416, Message:"No database selected", State:"3D000"} ("ERROR 1046 (3D000): No database selected")

OOPS: 18 passed, 3 FAILED, 1 PANICKED
--- FAIL: Test (0.46s)
FAIL
exit status 1
FAIL	github.com/go-mysql-org/go-mysql/client	0.465s
```

Author: dveeden

Makefile

@@ -11,7 +11,7 @@ build:
 test:
 	go test --race -timeout 2m ./...
 
-MYSQL_VERSION ?= 5.7
+MYSQL_VERSION ?= 8.0
 test-local:
 	docker run --rm -d --network=host --name go-mysql-server \
 		-e MYSQL_ALLOW_EMPTY_PASSWORD=true \

client/conn_test.go

@@ -97,6 +97,9 @@ func (s *connTestSuite) TestExecuteMultiple(c *C) {
 		`THIS IS BOGUS()`,
 	}
 
+	_, err := s.c.Execute("USE " + *testDB)
+	c.Assert(err, IsNil)
+
 	count := 0
 	result, err := s.c.ExecuteMultiple(strings.Join(queries, "; "), func(result *mysql.Result, err error) {
 		switch count {

docker/resources/waitfor.sh

@@ -4,7 +4,7 @@ port=$2
 
 echo "Waiting for mysql at $host:$port"
 while true; do
-  docker run --rm -it --network=host mysql:5.7 mysql -h$host -P$port -e "SELECT RAND()" >/dev/null
+  docker run --rm -it --network=host mysql:8.0 mysql -h$host -P$port -e "SELECT RAND()" >/dev/null
   if [[ $? -eq 0 ]]; then
     echo 'Connected'
     break