fix(cmd_duration): Make render_time format more consistent

[LionC]

Description

• Make timing format always show enabled components after te\he first nonzero component (see Make render_time format more consistent #5822 )
• Reduce the dynamic string allocation part of the render_time function to a single string allocation by pre-allocating the (pretty much fixed size for a given input) string
• Make render_time return 0s instead of 0ms when show_millis is false (that result is not rendered anyways though)
• Prefix the tests for the render_time function with render_time_ to make them runnable via cargo test render_time

Motivation and Context

Closes #5822

How Has This Been Tested?

• Passed the old tests except the ones it shouldn't
• Added new tests

Checklist:

• I have updated the documentation accordingly. (n/a)
• I have updated the tests accordingly.