Clean up SpeedTest output, avoid div-by-0

[earlephilhower]

Use a formatting function to give more human-readable output formats
for flash bandwidth. When the test starts and ends in less than one
millisecond, report as "Infinite".

Fixes #8339

Sample output:

Creating 512KB file, may take a while...
==> Time to write 512KB in 256b chunks = 6641 milliseconds
==> Created file size = 524288
Reading 512KB file sequentially in 256b chunks
==> Time to read 512KB sequentially in 256b chunks = 211 milliseconds = 2.48 MB/s
Reading 512KB file MISALIGNED in flash and RAM sequentially in 256b chunks
==> Time to read 512KB sequentially MISALIGNED in flash and RAM in 256b chunks = 212 milliseconds = 2.47 MB/s
Reading 512KB file in reverse by 256b chunks
==> Time to read 512KB in reverse in 256b chunks = 367 milliseconds = 1.43 MB/s
Writing 64K file in 1-byte chunks
==> Time to write 64KB in 1b chunks = 1249 milliseconds = 52.47 kB/s
Reading 64K file in 1-byte chunks
==> Time to read 64KB in 1b chunks = 296 milliseconds = 221.41 kB/s