Use const for suite (#739)

Author: tkf

benchmark/bench_qr.jl

@@ -4,7 +4,7 @@ using BenchmarkTools
 using LinearAlgebra
 using StaticArrays
 
-suite = BenchmarkGroup()
+const suite = BenchmarkGroup()
 
 for K = 1:22
     a = rand(SMatrix{K,K,Float64,K*K})

benchmark/benchmarks.jl

@@ -3,7 +3,7 @@
 # to the top-level group `SUITE` with the `$name` extracted from the file name.
 
 using BenchmarkTools
-SUITE = BenchmarkGroup()
+const SUITE = BenchmarkGroup()
 for file in sort(readdir(@__DIR__))
     if startswith(file, "bench_") && endswith(file, ".jl")
         SUITE[chop(file, head = length("bench_"), tail = length(".jl"))] =