[PHPLIB-1608] Implement prose tests for $lookup support

[paulinevos]

As laid out in the spec: mongodb/specifications@527e22d

Closes PHPLIB-1608

[codecov-commenter]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
101595	1	101594	19105

View the full list of 1 ❄️ flaky tests

MongoDB.Tests.UnifiedSpecTests.UnifiedSpecTest::testSessions with data set "sessions/implicit sessions default causal consistency: afterClusterTime is not sent on retried read in implicit session when readConcern level is snapshot"

Flake rate in main: 10.53% (Passed 17 times, Failed 2 times)

Stack Traces | 0.741s run time

MongoDB\Tests\UnifiedSpecTests\UnifiedSpecTest::testSessions with data set "sessions/implicit sessions default causal consistency: afterClusterTime is not sent on retried read in implicit session when readConcern level is snapshot"
Failed asserting that expected value matches actual value.
$actual count is 0, expected 1
--- Expected
+++ Actual
@@ @@
-MongoDB\Model\BSONArray Object #118360 (
-    0 => MongoDB\Model\BSONDocument Object #118414 (
-        '_id' => 1,
-        'x' => 'snapshot',
-    ),
-)
+MongoDB\Model\BSONArray Object #118401 ()

.../UnifiedSpecTests/Constraint/Matches.php:112
.../tests/UnifiedSpecTests/ExpectedResult.php:48
.../tests/UnifiedSpecTests/Operation.php:164
.../tests/UnifiedSpecTests/UnifiedTestRunner.php:213
.../tests/UnifiedSpecTests/UnifiedTestRunner.php:105
.../tests/UnifiedSpecTests/UnifiedSpecTest.php:286

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[GromNaN]

The tests match the prose tests. Good work.

[GromNaN]

Suggested change

[GromNaN]

1 CS fix and it's all good.