Generate "crdb-base" directory in target, not temporary directory

[smklein]

This new behavior abides by the rules for generating output from a build script, by using the OUT_DIR path available at compile-time.

To ensure that the OUT_DIR path is available to the build script and invocations of test_setup_database, the crates are organized as follows:

• test-utils, the top-level crate, contains functionality for both populating + copying from a seed directory. This is made available to both the build script (in nexus/test-utils) and any tests (in nexus) that want to call test_setup_database. Calling the test-utils version of test_setup_database now explicitly requires providing a path to the seed directory.
• The build script within nexus/test-utils defines an OUT_DIR, and creates the crdb-base directory there. It additionally provides another implementation of test_setup_database, which also embeds the path to the crdb-base directory.
• Most tests use the version of test_setup_database from nexus/test-utils, which embeds the path to crdb-base created from the build script.

Fixes #548

[davepacheco]

Thanks for doing this!