Since PHP 7, `assert()` would throw an `AssertionError` if the ini
option `assert.exception` was enabled, but it didn't become the default
until PHP 8 so instead warnings were converted to an `ErrorException`.

This commit updates phpunit's bootstrap script to make sure that the
`assert.exception` option is enabled for both PHP 7 *and* 8, enabling us
to switch the expectation to a proper `AssertionError` in the tests.

Bumps infection, phpunit and php-invoker to the latest versions to get
support for php8 and the xdebug config changes.

With the update to phpunit comes a new xml config schema. The new
`coverage` section is made up of the `filter.whitelist` value (now
'include') and the `log.coverage-*` values as children of `report`.

After updating the upstream image to include pcntl, Infection PHP
flagged a mutation in `Collection\CollectionTuple::__toArray()` where
`$this->value instanceof ShellInterface` could be changed to `false`.

That results in the first half of the ternary never executing. To catch
it, this commit adds a test for `__toArray()` that uses an instance of
`ShellBuilder`, complimenting the existing string-based test.

After updating psalm, 17 errors were uncovered - all either a
MissingConstructor error or PropertyNotSetInConstructor - largely
depending on whether it found it in the `final` class or a parent.

In all cases, the root cause was simply lack of a default value.

The most recent version of Infection PHP that will run on 7.2 is 0.15.x.
v0.19 adds support for php 8, but also drops support for versions older
than 7.4 meaning we also need to allow for 0.18.x to get 7.3 working.

Phpunit 9 won't run under 7.2, but phpunit 8 doesn't understand the
config for v9. This here is an attempt at a "dual boot" of sorts.

The old config is restored to a separate file so we can pass the legacy
config for 7.2 but still use the new config format for other versions.

Manually installs the latest copy of phpunit 8.x for php 7.3 because
while phpunit9 is compatible with php7.3, infection is not able to read
the new xml config schema that phpunit uses as of v9.

Copies the legacy config rather than moving it so that xmllint can scan
it, which was the last thing preventing the 7.x build from passing.

Since the config is always available under the same filename, we also no
longer need to pass a custom `--configuration` flag for phpunit.

For php 8, we need to manually set the new `xdebug.mode` option to
enable coverage mode, but it won't work if we simply do something like
`php -d xdebug.mode=coverage composer infection` because InfectionPHP
spawns new processes that would lose that option.

To provide for this, there's the `--initial-tests-php-options` shell
option to infection that will pass that on when it calls PHP for the
initial test run, which is the only time we need coverage enabled.

Since this is only required in PHP 8 and there are no other parts using
the `PHPUNIT_LEGACY` env variable, we can instead test against php 7.x
to remove the need for PHPUNIT_LEGACY entirely.

Infection PHP hasn't released the commit adding php 8 support yet, so
for now we reference it directly as a fallback with 1.24 preemptively
set ready for when it's actually available.

phpro/grumphp still requires ^7.3 so we can't stop ignoring platform
requirements just yet, but enforcing 4.0.1 explicitly at least removes
the error about 4.0.0 requiring the composer v1 plugin api package.

Co-authored-by: Chris Ben <chris.ben9@gmail.com>

Add support for PHP 8 closes #17