test/bench: add bcast benchmark

[hzhou]

Pull Request Description

Following #6907, this PR adds collective benchmarks, starting with bcast.

On my desktop:

$ export MPITEST_VERBOSE=1
$ mpirun -l -n 8 ./bcast -memtype=device
[7] Allocating buffer: memtype=device, device=7, size=5000000
[0] Allocating buffer: memtype=device, device=0, size=5000000
[0] TEST bcast:
[6] Allocating buffer: memtype=device, device=6, size=5000000
[2] Allocating buffer: memtype=device, device=2, size=5000000
[3] Allocating buffer: memtype=device, device=3, size=5000000
[5] Allocating buffer: memtype=device, device=5, size=5000000
[1] Allocating buffer: memtype=device, device=1, size=5000000
[4] Allocating buffer: memtype=device, device=4, size=5000000
[0] Barrier latency 33.098 +/- 0.743 us
[0]            1    283.896      4.881            0.004
[0]            2    282.667      0.864            0.007
[0]            4    282.172      0.955            0.014
[0]            8    282.547      0.987            0.028
[0]           16    281.973      0.960            0.057
[0]           32    258.795      0.529            0.124
[0]           64    258.640      1.185            0.247
[0]          128    258.742      0.917            0.495
[0]          256    288.946      3.025            0.886
[0]          512    275.079     10.361            1.861
[0]         1024    278.241      3.867            3.680
[0]         2048    276.950      3.892            7.395
[0]         4096    279.772      4.040           14.641
[0]         8192    284.587      3.501           28.786
[0]        16384    293.846      5.354           55.757
[0]        32768    112.957      2.848          290.093
[0]        65536    176.461     26.542          371.392
[0]       131072    279.492     12.952          468.965
[0]       262144    471.296     17.447          556.219
[0]       524288   1091.045     17.324          480.537
[0]      1048576   2338.931     39.098          448.314
[0]      2097152   3355.044     36.571          625.074
[0]      4194304   6950.164     71.623          603.483
[0]
[0]  No Errors[0]

[skip warnings]

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