Improve BenchmarkText_lineBounds_xxx

[MaxGyver83]

Description:

Reset the font metrics cache before every lineBounds benchmark run. Otherwise, the first run is slow and all further are fast because they can use the cached measurements of the first run.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 60.4% (-0.4%) from 60.808%
when pulling f412e4d on MaxGyver83:improve-BenchmarkText_lineBounds-benchmarks
into 33a4ea1 on fyne-io:develop.