Change the way unit tests are built and run

[brson]

r? @graydon

This series changes the build such that libraries are not completely rebuilt when testing. Instead, the tests are built into the libraries, and driver.rs is modified to run the tests directly out of them. It should make cycle times faster.

This will add some binary size to the libraries, but I figure we can have a 'release mode' some day that doesn't build them. It will also slightly increase the normal, non-testing, build times.

To make this work I modified rustc::front::test to fold in the test modifications any time the 'test' cfg is active, allowing the test modifications to be performed on libraries in addition to executables (previously this was only done with the --test flag, which turns on both --cfg test and --bin).

Measurements forthcoming.

[brson]

One of these patches also adds test targets for the syntax crate, which had two bitrotted tests that I removed.

[brson]

Oh, as a side benefit this removes the wierd testing situation around core that makes using TLS and defining lang items difficult. Required moving just three type definitions from std::test to core::private::test.

[brson]

Fixes #2912

[brson]

Here are some numbers. the First two show that time for running tests (with make already having run) went down by about 3 minutes. The last two show that time to actually run make, without running tests, went up by nearly 1m30s.

before, make check, prebuilt

real 16m21.570s
user 71m42.569s
sys 2m17.165s

after, make check, prebuilt

real 13m35.560s
user 73m38.548s
sys 2m23.973s

before, make

real 8m6.354s
user 7m49.689s
sys 0m18.841s

after, make

real 9m32.133s
user 9m12.227s
sys 0m21.497s

[brson]

Here are some better numbers. This is a common development scenario, where you've made a change to std and want to test it. It takes about half as long now since as soon as std is built it can be tested.

before, make check-stage1-std, prebuilt but with modifications to std

real 5m11.610s
user 4m29.913s
sys 0m8.821s

after, make check-stage1-std, prebuilt but with modifications to std

real 2m30.152s
user 2m24.773s
sys 0m5.524s

[brson]

This results in massive increases in binary size. core goes from 2.8M to 9.9M

[brson]

I merged the independently useful commits. Shelving this idea for now.