Tidy up benchmarks.jl, add option to select suites

Example of selecting a suite:
```julia
benchmarkpkg(GCPDecompositions, BenchmarkConfig(;env=Dict("GCP_BENCHMARK_SUITES"=>"mttkrp")))
```

Inspired by: https://github.com/JuliaCI/BaseBenchmarks.jl/blob/0ec1edace2d731de8d926f4b48a22cf2e5cc99c2/src/BaseBenchmarks.jl

benchmark/benchmarks.jl

@@ -1,16 +1,18 @@
 using BenchmarkTools
 
-BenchmarkTools.DEFAULT_PARAMETERS.seconds = 1
+# Default benchmark parameters
+BenchmarkTools.DEFAULT_PARAMETERS.seconds = 1.0
 
-const SUITE = BenchmarkGroup()
-
-module BenchmarkGCP
-    include("gcp.jl")
-end
+# Benchmark suite modules
+const SUITE_MODULES = Dict("gcp" => :BenchmarkGCP, "mttkrp" => :BenchmarkMTTKRP)
+const DEFAULT_SUITES = join(keys(SUITE_MODULES), ' ')
 
-module BenchmarkMTTKRP
-    include("mttkrp.jl")
+# Create top-level suite including only sub-suites
+# specified by ENV variable "GCP_BENCHMARK_SUITES"
+const SUITE = BenchmarkGroup()
+SELECTED_SUITES = split(get(ENV, "GCP_BENCHMARK_SUITES", DEFAULT_SUITES))
+for suite_name in SELECTED_SUITES
+    module_name = SUITE_MODULES[suite_name]
+    include(joinpath(@__DIR__, "$(suite_name).jl"))
+    SUITE[suite_name] = eval(module_name).SUITE
 end
-
-SUITE["gcp"] = BenchmarkGCP.SUITE
-SUITE["mttkrp"] = BenchmarkMTTKRP.SUITE

benchmark/gcp.jl

@@ -1,3 +1,5 @@
+module BenchmarkGCP
+
 using BenchmarkTools
 
 using Random
@@ -40,3 +42,5 @@ for sz in [(15, 20, 25), (30, 40, 50)], r in 1:2
     X = [rand(Bernoulli(M[I]/(M[I] + 1))) for I in CartesianIndices(size(M))]
     bench_gcp["bernoulliOdds-size(X)=$sz, rank(X)=$r"] = @benchmarkable gcp($X, $r, BernoulliOddsLoss())
 end
+
+end

benchmark/mttkrp.jl

@@ -1,3 +1,5 @@
+module BenchmarkMTTKRP
+
 using BenchmarkTools
 
 using Random
@@ -17,4 +19,6 @@ for sz in shapes, r in rs
     X = randn(sz)
     U = [randn(Ik,r) for Ik in sz]
     bench_mttkrp["mttkrp-size(X)=$sz, rank(X)=$r"] = @benchmarkable GCPDecompositions.mttkrp($X, $U, $n)
+end
+
 end