command/export: report which files are being written
Remove "test_awkward" integration test

The functionality this tests is already amply covered by unit tests of
Process and Printer, and the integration test is much more fragile.
Remove "test_env" integration tests

These tests are either a) testing things that aren't honcho's
responsibility, such as the OS shell's interpolation of environment
variables, or b) testing things that are already better covered by unit
tests of the `honcho.environ` package.
Add a complete TestEnv context manager to helpers

This can be used to set up a directory in which to test an instance of
honcho, without relying on the contents of the fixtures directory, which
is getting unwieldy.
Simplify export integration tests
Simplify check integration tests
Remove unnecessary concurrency integration test

This test was broken and is unnecessary anyway. It's testing argparse.
Simplify run integration tests
Remove unnecessary integration test

This functionality is already covered by test_write_invalid_utf8 in
honcho.test.unit.test_printer.
Simplify start integration tests
test: Remove unused helper functions