test: fix phpunit-9.1 warnings

[Totktonada]

There were 35 warnings:
1) DMLTest::test_16_hash_select
assertRegExp() is deprecated and will be removed in PHPUnit 10. Refactor your code to use assertMatchesRegularExpression() instead.
2) DMLTest::test_17_01_it_clienterror with data set #0 ('test_hash', 'EQ', '(Invalid key part|via a partial key)')
assertRegExp() is deprecated and will be removed in PHPUnit 10. Refactor your code to use assertMatchesRegularExpression() instead.

https://github.com/sebastianbergmann/phpunit/blob/7d1520684e8c951c6f1601f70643946997b77899/ChangeLog-9.1.md#910---2020-04-03