Fix default arrow build

[alamb]

Which issue does this PR close?

Built on #532 so review that one first

Closes #529

Rationale for this change

Arrow does not build today unless rand 0.8 is included as another dependency of the project; I introduced this in #488 when I upgraded the rand dependency but disabled the default features (which require sys which mean arrow can't be built on webasm anymore)

Since the only use of the sys part of the rand dependency is in benchmarks and testing code, I propose simply not building that part of of the arrow crate if default features are turned off

What changes are included in this PR?

1. Bring enough of the rand crate in by default to build, and #ifdef out their use when arrows default features are disabled

Are there any user-facing changes?

There is a test_utils feature flag now, enabled by default, that gates what parts of arrow are compiled if the default features are turned off

[alamb]

It would be nice to have all this test only code somewhere that wasn't the actual arrow crate at all, but since the test code has a backwards dependency on arrow -- (arrow <--> test_util) -- we would have to break that cycle as well, which is a lot of code churn

[alamb]

FYI @ritchie46 and @joshuataylor

[codecov-commenter]

Codecov Report

Merging #533 (abd8cfe) into master (f1fb2b1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #533   +/-   ##
=======================================
  Coverage   82.60%   82.60%           
=======================================
  Files         167      167           
  Lines       45984    45984           
=======================================
  Hits        37984    37984           
  Misses       8000     8000           

Impacted Files	Coverage Δ
arrow/src/util/bit_util.rs	100.00% <ø> (ø)

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[joshuataylor]

I have tested this by removing rand from my Cargo.toml and then changed arrow to this:

arrow = { git = "https://github.com/alamb/arrow-rs", branch = "alamb/fix-arrow-default-build" }

And everything works well. 👍