Move COMPILER_TESTS.md out of the root directory

src/test/COMPILER_TESTS.md

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+# Compiler Test Documentation
+
+In the Rust project, we use a special set of commands embedded in
+comments to test the Rust compiler. There are two groups of commands:
+
+1. Header commands
+2. Error info commands
+
+Both types of commands are inside comments, but header commands should
+be in a comment before any code.
+
+## Summary of Error Info Commands
+
+Error commands specify something about certain lines of the
+program. They tell the test what kind of error and what message you
+are expecting.
+
+* `~`: Associates the following error level and message with the
+  current line
+* `~|`: Associates the following error level and message with the same
+  line as the previous comment
+* `~^`: Associates the following error level and message with the
+  previous line. Each caret (`^`) that you add adds a line to this, so
+  `~^^^^^^^` is seven lines up.
+
+The error levels that you can have are:
+
+1. `ERROR`
+2. `WARNING`
+3. `NOTE`
+4. `HELP` and `SUGGESTION`*
+
+\* **Note**: `SUGGESTION` must follow immediately after `HELP`.
+
+## Summary of Header Commands
+
+Header commands specify something about the entire test file as a
+whole, instead of just a few lines inside the test.
+
+* `ignore-X` where `X` is an architecture, OS or stage will ignore the test accordingly
+* `ignore-pretty` will not compile the pretty-printed test (this is done to test the pretty-printer, but might not always work)
+* `ignore-test` always ignores the test
+* `ignore-lldb` and `ignore-gdb` will skip the debuginfo tests
+* `min-{gdb,lldb}-version`
+* `should-fail` indicates that the test should fail; used for "meta testing",
+  where we test the compiletest program itself to check that it will generate
+  errors in appropriate scenarios. This header is ignored for pretty-printer tests.
+* `gate-test-X` where `X` is a feature marks the test as "gate test" for feature X.
+  Such tests are supposed to ensure that the compiler errors when usage of a gated
+  feature is attempted without the proper `#![feature(X)]` tag.
+  Each unstable lang feature is required to have a gate test.
+
+## Revisions
+
+Certain classes of tests support "revisions" (as of the time of this
+writing, this includes run-pass, compile-fail, run-fail, and
+incremental, though incremental tests are somewhat
+different). Revisions allow a single test file to be used for multiple
+tests. This is done by adding a special header at the top of the file:
+
+```
+// revisions: foo bar baz
+```
+
+This will result in the test being compiled (and tested) three times,
+once with `--cfg foo`, once with `--cfg bar`, and once with `--cfg
+baz`. You can therefore use `#[cfg(foo)]` etc within the test to tweak
+each of these results.
+
+You can also customize headers and expected error messages to a particular
+revision. To do this, add `[foo]` (or `bar`, `baz`, etc) after the `//`
+comment, like so:
+
+```
+// A flag to pass in only for cfg `foo`:
+//[foo]compile-flags: -Z verbose
+
+#[cfg(foo)]
+fn test_foo() {
+    let x: usize = 32_u32; //[foo]~ ERROR mismatched types
+}
+```
+
+Note that not all headers have meaning when customized to a revision.
+For example, the `ignore-test` header (and all "ignore" headers)
+currently only apply to the test as a whole, not to particular
+revisions. The only headers that are intended to really work when
+customized to a revision are error patterns and compiler flags.