Git ignore test performance reports (#768)

### Description
As a result of running the tests a file with performance information is
created. These files should not be included in any commit so I add them
to .gitignore.

### Checklist
- [x] Code compiles correctly
- [x] Code is formatted according to [Coding
Conventions](../CodingConventions.md)
- [x] Created tests which fail without the change (if possible)
- [x] All tests passing (`dotnet test`)
- [x] Extended the README / documentation, if necessary
- [x] Provide JIRA issue id (if possible) or GitHub issue id in PR name

.gitignore

@@ -304,3 +304,8 @@ wss-unified-agent.jar
 whitesource/
 /testEnvironments.json
 /parameters.json
+
+# Test performance reports
+Snowflake.Data.Tests/macos_*_performance.csv
+Snowflake.Data.Tests/windows_*_performance.csv
+Snowflake.Data.Tests/unix_*_performance.csv