apacheGH-37187: [MATLAB] Re-implement tfeathermex.m tests in terms …

…of new internal Feather Reader and Writer objects (apache#37189)

### Rationale for this change

Now that we have new internal Feather V1 `arrow.internal.io.feather.Reader` and `arrow.internal.io.feather.Writer` objects, we can re-implement the `tfeathermex.m` tests in terms of calls to these new internal APIs. As part of this change, we can also move these test cases into `matlab/test/arrow/io/feather/tRoundTrip.m` and delete the old `tfeathermex.m` file. Deleting the old Feather MEX tests will allow us to also delete the old Feather MEX code.

### What changes are included in this PR?

1. Moved test cases `NumericDatatypesNulls` and `InvalidMATLABTableVariableNames` from `test/tfeathermex.m` to `test/arrow/internal/io/feather/tRoundTrip.m`
2. Deleted `test/tfeathermex.m`
3. Deleted obsolete test utility `test/util/createVariablesAndMetadataStructs.m`
4. Deleted obsolete test utility `test/util/featherMEXRoundTrip.m`

### Are these changes tested?

Yes. All tests pass.

### Are there any user-facing changes?

No.

### Future Directions

1. Delete the MEX source code from the interface.
2. Move `test/util/createTable.m` and `test/util/featherRoundTrip.m` into package functions underneath `arrow.internal.test.*` (See apache#37188)
* Closes: apache#37187

Authored-by: Sarah Gilmore <sgilmore@mathworks.com>
Signed-off-by: Kevin Gurney <kgurney@mathworks.com>

matlab/test/arrow/io/feather/tRoundTrip.m

@@ -52,6 +52,70 @@ function Basic(testCase)
 
             testCase.verifyEqual(tableWrite, tableRead);
         end
-    end
 
+        function NumericDatatypesNulls(testCase)
+            % Verify integers with null values are roundtripped correctly.
+            import matlab.unittest.fixtures.TemporaryFolderFixture
+            import arrow.internal.io.feather.*
+            import arrow.tabular.RecordBatch
+
+            fixture = testCase.applyFixture(TemporaryFolderFixture);
+            filename = fullfile(fixture.Folder, "temp.feather");
+
+            uint8Array = arrow.array(uint8([1, 2, 3, 4, 5]), Valid=[1, 3, 4]);
+            int16Array = arrow.array(uint8([1, 2, 3, 4, 5]), Valid=[3, 5]);
+            inputRecordBatch = RecordBatch.fromArrays(uint8Array, int16Array);
+
+            writer = Writer(filename);
+            writer.write(inputRecordBatch);
+
+            reader = Reader(filename);
+            outputRecordBatch = reader.read();
+
+            testCase.verifyEqual(outputRecordBatch.NumColumns, inputRecordBatch.NumColumns);
+            testCase.verifyEqual(outputRecordBatch.ColumnNames, inputRecordBatch.ColumnNames);
+
+            outColumn1 = outputRecordBatch.column(1);
+            testCase.verifyEqual(outColumn1.Valid, uint8Array.Valid);
+            testCase.verifyEqual(toMATLAB(outColumn1), toMATLAB(uint8Array));
+
+            outColumn2 = outputRecordBatch.column(2);
+            testCase.verifyEqual(outColumn2.Valid, int16Array.Valid);
+            testCase.verifyEqual(toMATLAB(outColumn2), toMATLAB(int16Array));
+        end
+
+        function InvalidMATLABTableVariableNames(testCase)
+            import matlab.unittest.fixtures.TemporaryFolderFixture
+            import arrow.internal.io.feather.*
+            import arrow.tabular.RecordBatch
+
+            fixture = testCase.applyFixture(TemporaryFolderFixture);
+            filename = fullfile(fixture.Folder, "temp.feather");
+
+            doubleArray = arrow.array([1, 2, 3, 4, 5]); 
+            stringArray = arrow.array(["A", "B", "C", "D", "E"]);
+
+            % Create a RecordBatch whose first column name is an invalid
+            % variable name for a MATLAB table.
+            inputRecordBatch = RecordBatch.fromArrays(doubleArray, stringArray, ...
+                ColumnNames=[":", "Valid"]);
+
+            writer = Writer(filename);
+            writer.write(inputRecordBatch);
+
+            % Verify the RecordBatch returned by read has the same
+            % ColumnNames as the original RecordBatch.
+            reader = Reader(filename);
+            outputRecordBatch = reader.read();
+            testCase.verifyEqual(outputRecordBatch.ColumnNames, inputRecordBatch.ColumnNames);
+
+            % Verify the VariableNames property in the table returned by
+            % featherread has been modified and that 
+            % VariableDescriptions is set property.
+            outputTable = featherread(filename);
+            testCase.verifyEqual(outputTable.Properties.VariableNames, {':_1', 'Valid'});
+            testCase.verifyEqual(outputTable.Properties.VariableDescriptions, ...
+                {'Original variable name: '':''', ''});
+        end
+    end
 end