use phpunit 9

This should be the last version supporting php 7.4

Also allow test files ending in Test.php

This makes it possible to give test files proper PSR-2 names where the
file is named after the class name.

support phpunit 5 and 6

We still need 5 to test on old php versions, but travis uses 6 for newer
PHP versions (and 5 may break on those).

Luckily both versions aren't that far apart, yet. This adds

support phpunit 5 and 6

We still need 5 to test on old php versions, but travis uses 6 for newer
PHP versions (and 5 may break on those).

Luckily both versions aren't that far apart, yet. This adds a thin
adapter class when running on 6.

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introduce new "flaky" test group

this group is skipped by default

some updates on phpunit docs and settings

allow template unit tests

only include dw files in coverage report except _ directories

for consistency, also use _test directory for plugins

moved _testing to _test