Document xml-report and tail-lines for mysql-test-run

 xml-report=<file>     Output jUnit xml file of the results.

$ ./mtr alias --xml-report=mtr.xml

$ cat mysql-test/mtr.xml

<?xml version="1.0" encoding="UTF-8"?>

<testsuites disabled="0" errors="" failures="0" name="" tests="1" time="0.040">

	<testsuite disabled="" errors="" failures="" hostname="anel" id="0" name="main" package="" skipped="" tests="1" time="0.040" timestamp="2021-02-01T10:03:01Z">

		<testcase assertions="" classname="main" name="alias" status="MTR_RES_PASSED" time="0.040" />

	</testsuite>

</testsuites>

tail-lines=N          Number of lines of the result to include in a failure report.

$ ./mysql-test/mtr alias # failure after <snip> there are 20 lines (default)

The result from queries just before the failure was:

< snip >

(select t2.* from t2)

99

99

select a, (select t2.* from t2) from t1;

a	(select t2.* from t2)

1	99

11	99

select t1.*, (select t2.* from t2) from t1;

a	b	c	(select t2.* from t2)

1	2	3	99

11	22	33	99

insert into t3 select t1.* from t1;

insert into t3 select t2.*, 1, 2 from t2;

insert into t3 select t2.*, d as 'x', d as 'z' from t2;

insert into t3 select t2.*, t2.*, 3 from t2;

create table t4 select t1.* from t1;

drop table t4;

create table t4 select t2.*, 1, 2 from t2;

drop table t4;

create table t4 select t2.*, d as 'x', d as 'z' from tt;

$ ./mysql-test/mtr alias --tail-lines=3 # failure after <snip> there are 3 lines (as set)

The result from queries just before the failure was:

< snip >

create table t4 select t2.*, 1, 2 from t2;

drop table t4;

create table t4 select t2.*, d as 'x', d as 'z' from tt;