[lit] Keep stdout/stderr when using GoogleTest format

[lit] Keep stdout/stderr when using GoogleTest format

When a unit test crashes or timeout, print the shard's stdout and
stderr. When a unit test fails, attaches the test's output to the LIT
output to help debugging.

While at it, concatenating shard's environment variables using space
instead of newline to make the reproducer script user friendly.

Based on D123797. (Thanks to @lenary)