As discussed in #26, the changes in go1.21 will remove rand contention
when the (now deprecated) rand.Seed() is not called.

In order to check if this brings performance parity, start to benchmark
both Pick and PickSource functions in parallel, and no longer rand.Seed
in test init.

results on my Apple M2 Pro laptop:

$ make analysis
benchstat -filter=".name:Multiple" -col="/lib" -row="/size" -table="/concurrency" results.txt
/concurrency: single
        │    randutil     │             weightedrand             │
        │     sec/op      │   sec/op     vs base                 │
1e1          455.70n ± 0%   19.16n ± 2%   -95.80% (p=0.000 n=10)
1e2          438.20n ± 0%   32.81n ± 0%   -92.51% (p=0.000 n=10)
1e3         1179.00n ± 0%   46.00n ± 0%   -96.10% (p=0.000 n=10)
1e4         6591.00n ± 1%   60.34n ± 0%   -99.08% (p=0.000 n=10)
1e5        61188.00n ± 0%   84.83n ± 0%   -99.86% (p=0.000 n=10)
1e6        631928.5n ± 1%   112.6n ± 2%   -99.98% (p=0.000 n=10)
1e7       6402254.0n ± 1%   240.7n ± 0%  -100.00% (p=0.000 n=10)
geomean       16.84µ        63.16n        -99.62%

/concurrency: parallel
        │     randutil     │             weightedrand             │
        │      sec/op      │   sec/op     vs base                 │
1e1          589.300n ± 0%   2.026n ± 1%   -99.66% (p=0.000 n=10)
1e2          490.000n ± 0%   3.337n ± 1%   -99.32% (p=0.000 n=10)
1e3          766.500n ± 0%   4.518n ± 1%   -99.41% (p=0.000 n=10)
1e4          808.700n ± 0%   5.951n ± 2%   -99.26% (p=0.000 n=10)
1e5         6438.500n ± 1%   8.184n ± 1%   -99.87% (p=0.000 n=10)
1e6        108285.50n ± 1%   12.00n ± 1%   -99.99% (p=0.000 n=10)
1e7       2011263.00n ± 2%   28.95n ± 1%  -100.00% (p=0.000 n=10)
geomean        5.907µ        6.548n        -99.89%

key-value format is nicer for later filtering in benchstat