sync unified tests

Author: baileympearson

test/spec/gridfs/README.md

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+# GridFS Tests
+
+______________________________________________________________________
+
+## Introduction
+
+The YAML and JSON files in this directory are platform-independent tests meant to exercise a driver's implementation of
+GridFS. These tests utilize the [Unified Test Format](../../unified-test-format/unified-test-format.md).
+
+## Conventions for Expressing Binary Data
+
+The unified test format allows binary stream data to be expressed and matched with `$$hexBytes` (for uploads) and
+`$$matchesHexBytes` (for downloads), respectively; however, those operators are not supported in all contexts, such as
+`insertData` and `outcome`. When binary data must be expressed as a base64-encoded string
+([Extended JSON](../../extended-json.md) for a BSON binary type), the test SHOULD include a comment noting the
+equivalent value in hexadecimal for human-readability. For example:
+
+```yaml
+data: { $binary: { base64: "ESIzRA==", subType: "00" } } # hex 11223344
+```
+
+Creating the base64-encoded string for a sequence of hexadecimal bytes is left as an exercise to the developer. Consider
+the following PHP one-liner:
+
+```shell-session
+$ php -r 'echo base64_encode(hex2bin('11223344')), "\n";'
+ESIzRA==
+```