Auto merge of #19440 - asajeffrey:test-perf-utf8, r=jdm

Added explicit UTF-8 encoding to test-perf.

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Added explicit UTF-8 encoding to test-perf, to try to avoid:
```
Traceback (most recent call last):
  File "runner.py", line 371, in <module>
    main()
  File "runner.py", line 361, in main
    save_result_csv(results, args.output_file, testcases, args.runs, args.base)
  File "runner.py", line 287, in save_result_csv
    writer.writerows(results)
  File "/usr/lib/python3.4/csv.py", line 159, in writerows
    return self.writer.writerows(rows)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 101-107: ordinal not in range(128)
```
http://build.servo.org/builders/linux-nightly/builds/526/steps/test/logs/stdio

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is test infrastructure

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