CLDR-17938 Add test data generator for date time formatting (#4011)

common/testData/datetime/README.md

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+# Test Data for DateTime Formatting
+
+This directory contains test data for DateTime formatting.
+This data is the result of the test data generator `GenerateDateTimeTestData.java`.
+
+The design of the test generator is to provide the "expected value" of datetime formatting over several dimensions,
+such as locale,
+formatting options (date length, calendar, etc.), 
+and various input datetime values.
+
+The test generator constructs the expected value using the various pieces
+(date format pattern, time format pattern, datetime "glue" pattern)
+and `SimpleDateFormat`s to combine them together.
+Each test case reports the inputs for the test case and the expected value.