benchmark-matmul: Print the average of the test results

[rankaiyx]

The previous test results are too uncertain. It is difficult to record and compare, so add the ability to print the average of the test results.

[slaren]

• Fix duplicate whitespace, use 4 spaces indentation like everywhere else
• Put the code after the loop instead of checking if it is the last iteration

[rankaiyx]

• Fix duplicate whitespace, use 4 spaces indentation like everywhere else
• Put the code after the loop instead of checking if it is the last iteration

ok, I will do it.

[rankaiyx]

Thanks for your review. The pull request has been resubmitted.