MDEV-26715 Fix mysql_install_db_win tests.

It looks like  C/C reconnect retains the database. It is correct behavior
However there is a problem that mysql_install_db.exe does not create
'test' database, thus using mysql_install_db to create datadir,
then restarting server and reconnecting does not succeed, if 'test' was
current db before reconnect.

The problem became apparent with MDEV-26715. Why it worked before,
with a simpler 'root' definition is a mystery.

Fix is to switch current connection to 'mysql' database, prior to
reconnect to the instance created via mysql_install_db.exe

mysql-test/main/mysql_install_db_win.result

@@ -1,3 +1,7 @@
+select database();
+database()
+test
+use mysql;
 Running bootstrap
 Creating my.ini file
 Removing default user
@@ -29,3 +33,4 @@ skip-name-resolve
 [client]
 port=3307
 # restart
+use test;