test: Add an option to not capture output

A new flag to the test runner, --nocapture, can be passed to instruct that the
output of tests should not be captured by default. The behavior can also be
triggered via a RUST_TEST_NOCAPTURE environment variable being set.

Closes #13374
auto merge of #13706 : alexcrichton/rust/test-nocapture, r=brson

A new flag to the test runner, --nocapture, can be passed to instruct that the
output of tests should not be captured by default. The behavior can also be
triggered via a RUST_TEST_NOCAPTURE environment variable being set.

Closes #13374