Add VideoIO.framerate(f) to the manual and expand tests

[IanButterworth]

Related to #392

Todo:

• [x] Add expected values tests for example videos
• [x] Add generated video tests with more fps values

cc. @AbelHo

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (755ad16) 79.43% compared to head (7f2c717) 79.43%.

:exclamation: Current head 7f2c717 differs from pull request most recent head 7eb87fc. Consider uploading reports for the commit 7eb87fc to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #393 +/- ## ======================================= Coverage 79.43% 79.43% ======================================= Files 10 10 Lines 1279 1279 ======================================= Hits 1016 1016 Misses 263 263 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaIO/VideoIO.jl/pull/393?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/avio.jl](https://app.codecov.io/gh/JuliaIO/VideoIO.jl/pull/393?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2F2aW8uamw=) | `76.49% <100.00%> (+0.10%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaIO/VideoIO.jl/pull/393/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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[AbelHo]

results from this https://github.com/JuliaIO/VideoIO.jl/pull/393/commits/7f2c7173addc2f2cc31594bca4512bae8e7049e0 :

julia> VideoIO.framerate.(VideoIO.openvideo.(fnames))
5-element Vector{Rational{Int32}}:
 1//90000
 1//12800
 1//15360
 1//1000
 1//25000

julia> map(fnames) do x
               vid=VideoIO.openvideo(x); read(vid); VideoIO.framerate(vid)
       end
5-element Vector{Rational{Int32}}:
 1//90000
 1//12800
 1//15360
 1//1000
 1//25000

julia> get_fps.(fnames)
5-element Vector{Rational{Int64}}:
 30//1
 25//1
 30//1
 25//1
 25//1

[IanButterworth]

@AbelHo @f3fora would you mind reviewing this. I can't see why the tests are failing

[IanButterworth]

Bump @AbelHo (I saw the thumbs up. Thanks!)

[IanButterworth]

Bump. Help would be appreciated 🙏