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

+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;

mysql-test/main/mysql_install_db_win.test

 --source include/windows.inc
+select database();
+use mysql;
 
 # Create database  in tmp directory using mysql_install_db.exe, 
 # and start server from this directory.
@@ -87,4 +89,5 @@ perl;
 EOF
 rmdir $_BASEDIR;
 --source include/start_mysqld.inc
+use test;
 

mysql-test/main/mysql_install_db_win_utf8.result

+use mysql;
 Running bootstrap
 Creating my.ini file
 Removing default user
@@ -12,3 +13,4 @@ DATADIR/
 # Kill the server
 connection default;
 # restart
+use test;

mysql-test/main/mysql_install_db_win_utf8.test

 --source include/windows.inc
 --source include/check_utf8_cli.inc
-
+use mysql;
 # Create database  in tmp directory using mysql_install_db.exe,
 # and start server from this directory.
 let $ddir= $MYSQLTEST_VARDIR/tmp/датадир;
@@ -32,4 +32,5 @@ rmdir $ddir;
 
 connection default;
 --source include/start_mysqld.inc
+use test;
 

mysql-test/main/winservice.inc

@@ -9,7 +9,7 @@ let $sc_exe= C:\Windows\System32\sc.exe;
 let $ddir= $MYSQLTEST_VARDIR/tmp/$datadir_name;
 let $service_name=$service_name_prefix$MASTER_MYPORT;
 
-
+use mysql;
 error 0,1;
 rmdir $ddir;
 
@@ -72,4 +72,3 @@ rmdir $ddir;
 #restart original server
 connection default;
 source include/start_mysqld.inc;
-

mysql-test/main/winservice_basic.result

+use mysql;
 # run mysql_install_db with --service parameter
 # Start service
 # Connect with root user password=password

mysql-test/main/winservice_i18n.result

+use mysql;
 # run mysql_install_db with --service parameter
 # Start service
 # Connect with root user password=パスワード