Add measure of roundtrip performance for ArrayBuffers in coverall2 test

[jhugman]

This PR adds a test round tripping of up to 32 megabytes with millisecond timings.

The Linux box on CI shows:

ArrayBuffer roundtrip: 1 bytes in 2 ms
ArrayBuffer roundtrip: 2 bytes in 1 ms
ArrayBuffer roundtrip: 4 bytes in 0 ms
ArrayBuffer roundtrip: 8 bytes in 1 ms
ArrayBuffer roundtrip: 16 bytes in 1 ms
ArrayBuffer roundtrip: 32 bytes in 0 ms
ArrayBuffer roundtrip: 64 bytes in 0 ms
ArrayBuffer roundtrip: 128 bytes in 1 ms
ArrayBuffer roundtrip: 256 bytes in 1 ms
ArrayBuffer roundtrip: 512 bytes in 0 ms
ArrayBuffer roundtrip: 1 kB in 0 ms
ArrayBuffer roundtrip: 2 kB in 0 ms
ArrayBuffer roundtrip: 4 kB in 0 ms
ArrayBuffer roundtrip: 8 kB in 1 ms
ArrayBuffer roundtrip: 16 kB in 2 ms
ArrayBuffer roundtrip: 32 kB in 3 ms
ArrayBuffer roundtrip: 64 kB in 6 ms
ArrayBuffer roundtrip: 128 kB in 11 ms
ArrayBuffer roundtrip: 256 kB in 21 ms
ArrayBuffer roundtrip: 512 kB in 67 ms
ArrayBuffer roundtrip: 1 MB in 96 ms
ArrayBuffer roundtrip: 2 MB in 192 ms
ArrayBuffer roundtrip: 4 MB in 369 ms
ArrayBuffer roundtrip: 8 MB in 702 ms
ArrayBuffer roundtrip: 16 MB in 1368 ms
ArrayBuffer roundtrip: 32 MB in 2652 ms

My M3 MBP shows:

ArrayBuffer roundtrip: 1 bytes in 1 ms
ArrayBuffer roundtrip: 2 bytes in 0 ms
ArrayBuffer roundtrip: 4 bytes in 0 ms
ArrayBuffer roundtrip: 8 bytes in 1 ms
ArrayBuffer roundtrip: 16 bytes in 0 ms
ArrayBuffer roundtrip: 32 bytes in 0 ms
ArrayBuffer roundtrip: 64 bytes in 0 ms
ArrayBuffer roundtrip: 128 bytes in 0 ms
ArrayBuffer roundtrip: 256 bytes in 0 ms
ArrayBuffer roundtrip: 512 bytes in 0 ms
ArrayBuffer roundtrip: 1 kB in 0 ms
ArrayBuffer roundtrip: 2 kB in 0 ms
ArrayBuffer roundtrip: 4 kB in 1 ms
ArrayBuffer roundtrip: 8 kB in 0 ms
ArrayBuffer roundtrip: 16 kB in 1 ms
ArrayBuffer roundtrip: 32 kB in 1 ms
ArrayBuffer roundtrip: 64 kB in 3 ms
ArrayBuffer roundtrip: 128 kB in 7 ms
ArrayBuffer roundtrip: 256 kB in 13 ms
ArrayBuffer roundtrip: 512 kB in 43 ms
ArrayBuffer roundtrip: 1 MB in 59 ms
ArrayBuffer roundtrip: 2 MB in 114 ms
ArrayBuffer roundtrip: 4 MB in 245 ms
ArrayBuffer roundtrip: 8 MB in 429 ms
ArrayBuffer roundtrip: 16 MB in 838 ms
ArrayBuffer roundtrip: 32 MB in 1648 ms

Let's see what the device shows.

[zzorba]

LGTM