MDL-67673 phpunit: Introduce a new, lightweight phpunit_dataset

In charge of taking over the, now removed/archived, phpunit/dbunit
package. It supports loading of CSV/XML files and strings and
PHP arrays, allowing to send the loaded information to database.

Perform some basic controls about the consistency of information,
surely not super-exhaustive but fair enough.

100% covered with unit test.

Planned like an "interim" replacement for phpunit/dbunit uses in
core that, ideally, should be moved to generators stuff at some
point.

Note, expect a few tests in core to fail with this commit, I've
changed some fixtures around. Next commit will fix existing uses.

Originally MDL-64600

lib/phpunit/tests/fixtures/sample_dataset_wrong_attribute.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<dataset>
+    <table noname="user">
+        <column>id</column>
+        <column>username</column>
+        <column>email</column>
+        <row>
+            <value>5</value>
+            <value>pepa.novak</value>
+            <value>[email protected]</value>
+        </row>
+        <row>
+            <value>7</value>
+            <value>bozka.novakova</value>
+            <value>[email protected]</value>
+        </row>
+    </table>
+</dataset>