Apply JuliaFormatter with BlueStyle and add it to tests

[gdalle]

Other prominent JuliaCI repos like PkgTemplates.jl already use the BlueStyle. This PR does the same for BenchmarkTools.jl. More precisely, it

• sets up JuliaFormatter to use BlueStyle on this repo with the .JuliaFormatter.toml file
• applies automatic formatting to the existing code (mostly spacing modifications, all in the second commit)
• adds a test checking that formatting is correct in test/runtests.jl, along with a test-specific dependency in Project.toml
• adds a BlueStyle badge to README.md

[gdalle]

Fails because of https://github.com/domluna/JuliaFormatter.jl/issues/746