BenchmarkTools test fails on Windows

[strickek]

Package test fails on Windows because the needle in Regex only covers linux path-format.

This comes from BenchmarkTools.jl/test/ExecutionTests.jl: line 192-194

@test occursin(r"BenchmarkTools(\.jl)?/src/execution\.jl:\d+; _run", str)
@test !occursin(r"BenchmarkTools(\.jl)?/src/execution\.jl:\d+; warmup", str)
@test !occursin(r"BenchmarkTools(\.jl)?/src/execution\.jl:\d+; tune!", str)

I will provide a pull request.