MDL-71036 phpunit: Mock->setMethods() silently deprecated

The current ->setMethods() has been silently (won't emit any
warning) in PHPUnit 9. And will stop working (current plans)
in PHPUnit 10.

Basically the now deprecated method has been split into:

- onlyMethods(): To point to existing methods in the mocked artifact.
- addMethods(): To point to non existing (yet) methods in the mocked
  artifact.

In practice that means that all our current setMethods() calls can be
converted to onlyMethods() (existing) and done. The addMethods() is
mostly useful on development phases, not final testing.

Finally note that <null> isn't accepted anymore as parameter to
double all the methods. Instead empty array [] must be used.

Link: https://github.com/sebastianbergmann/phpunit/issues/3770

admin/tool/langimport/tests/locale_test.php

@@ -42,7 +42,7 @@ class locale_testcase extends \advanced_testcase {
         // - second setlocale() call which try to set new 'es' locale
         // - third setlocale() call which restore locale.
         $mock = $this->getMockBuilder(\tool_langimport\locale::class)
-            ->setMethods(['set_locale'])
+            ->onlyMethods(['set_locale'])
             ->getMock();
         $mock->method('set_locale')->will($this->onConsecutiveCalls('en', 'es', 'en'));
 
@@ -55,7 +55,7 @@ class locale_testcase extends \advanced_testcase {
         // - second setlocale() call which fail to set new locale
         // - third setlocale() call which restore locale.
         $mock = $this->getMockBuilder(\tool_langimport\locale::class)
-            ->setMethods(['set_locale'])
+            ->onlyMethods(['set_locale'])
             ->getMock();
         $mock->method('set_locale')->will($this->onConsecutiveCalls('en', false, 'en'));