Add a `harness` manifest option

This option is used to disable the --test flag to rustc for a test or benchmark
target in order to signal that the binary already knows how to run the testing
infrastructure.

The test/benchmark is still compiled and run as usual, and the exit code is
expected to reflect the result of the test/benchmark.

Closes #431
auto merge of #435 : alexcrichton/cargo/issue-431, r=brson

This option is used to disable the --test flag to rustc for a test or benchmark
target in order to signal that the binary already knows how to run the testing
infrastructure.

The test/benchmark is still compiled and run as usual, and the exit code is
expected to reflect the result of the test/benchmark.

Closes #431