Add a new src/test/pretty directory to hold just source files for testing the
pretty-printer.

Add a new pp-exact directive. When this directive is followed by a file name
it specifies a file containing the output that the pretty-printer should
generate. When pp-exact is not followed by a filename it says that the file
should pretty-print as written.

This will reduce the valgrind deluge when a test fails. The tests themselves
are still run under valgrind. Leave a CTEST_VALGRIND environment variable for
running with the old behavior.

This allows quicker turnaround for testing the standard library, since you
don't have to wait for the stage1 compiler every time.

This replaces the make-based test runner with a set of Rust-based test
runners. I believe that all existing functionality has been
preserved. The primary objective is to dogfood the Rust test
framework.

A few main things happen here:

1) The run-pass/lib-* tests are all moved into src/test/stdtest. This
is a standalone test crate intended for all standard library tests. It
compiles to build/test/stdtest.stageN.

2) rustc now compiles into yet another build artifact, this one a test
runner that runs any tests contained directly in the rustc crate. This
allows much more fine-grained unit testing of the compiler. It
compiles to build/test/rustctest.stageN.

3) There is a new custom test runner crate at src/test/compiletest
that reproduces all the functionality for running the compile-fail,
run-fail, run-pass and bench tests while integrating with Rust's test
framework. It compiles to build/test/compiletest.stageN.

4) The build rules have been completely changed to use the new test
runners, while also being less redundant, following the example of the
recent stageN.mk rewrite.

It adds two new features to the cfail/rfail/rpass/bench tests:

1) Tests can specify multiple 'error-pattern' directives which must be
satisfied in order.

2) Tests can specify a 'compile-flags' directive which will make the
test runner provide additional command line arguments to rustc.

There are some downsides, the primary being that Rust has to be
functioning pretty well just to run _any_ tests, which I imagine will
be the source of some frustration when the entire test suite
breaks. Will also cause some headaches during porting.

Not having individual make rules, each rpass, etc test no longer
remembers between runs whether it completed successfully. As a result,
it's not possible to incrementally fix multiple tests by just running
'make check', fixing a test, and repeating without re-running all the
tests contained in the test runner. Instead you can filter just the
tests you want to run by using the TESTNAME environment variable.

This also dispenses with the ability to run stage0 tests, but they
tended to be broken more often than not anyway.

This will link to std and compile with the --test flag. Eventually the
run-pass/lib* tests will move here.

We could also put the std tests directly into the library and compile both a
library version and a test version, but I think this way will make for faster
builds.

Issue #428

The tinderbox snapshotting scripts seem to be looking for a stage2-check
target when they should be using check-stage2. Since Graydon is out of the
office, I'm making this change to see if the snapshot will go through.

There's even more duplication now, but I'm reluctant to touch it since it's
easy to subtly break the build rules

rustc.exe.exe.

in getting a driver that works on all linux systems.

Sorry for the linker hacks, I will remove them after snapshotting a new compiler.

This is no longer necessary since error logging is not affected by RUST_LOG.

A typo in the Makefile was causing these to not run. Happily, a lot of these
tests pass, so un-xfailed.

Fix horrible rule-hijacking bug that was causing compile-fail targets to trigger honest compile attempts.