stage2 test harness: declarative format

[andrewrk]

It takes a long time to run ./zig build test-stage2 because it builds the full compiler. This issue is to introduce into the test harness, ability to load test cases from a declarative file format such as JSON, which references source files on disk. For example, here is the first test case from test/stage2/test.zig converted to declarative form:

manifest.json

{
  kind: "exe",
  target: "x86_64-linux",
  updates: {
    "empty file": {
      kind: "error",
      sources: {"test.zig": "empty.zig"},
      errors: [
        "test.zig:84:9: error: struct 'test_case.test_case' has no member named 'main'",
      ],
    },
    "incorrect return type": {
      kind: "error",
      sources: {"test.zig": "bad-start-ret-type.zig"},
      errors: [
        "test.zig:2:1: error: expected noreturn, found void",
      ],
    },
    "hello world bare bones": {
      kind: "compareOutput",
      sources: {"test.zig": "x86-asm-hello.zig"},
      stdout: "Hello, World!\n",
    },
    "convert to pub fn main": {
      kind: "compareOutput",
      sources: {"test.zig": "x86-asm-hello-2.zig"},
      stdout: "Hello, World!\n",
    },
    "change the message only": {
      kind: "compareOutput",
      sources: {"test.zig": "x86-asm-hello-3.zig"},
      stdout: "What is up? This is a longer message that will force the data to be relocated in virtual address space.\n",
    },
    "print it twice": {
      kind: "compareOutput",
      sources: {"test.zig": "x86-asm-hello-4.zig"},
      stdout: "What is up? This is a longer message that will force the data to be relocated in virtual address space.\nWhat is up? This is a longer message that will force the data to be relocated in virtual address space.\n",
    },
  }
}

file structure would look something like:

test/stage2/cases/hello world with updates/manifest.json
test/stage2/cases/hello world with updates/empty.zig
test/stage2/cases/hello world with updates/bad-start-ret-type.zig
test/stage2/cases/hello world with updates/x86-asm-hello.zig
test/stage2/cases/hello world with updates/x86-asm-hello-2.zig
test/stage2/cases/hello world with updates/x86-asm-hello-3.zig

On each update, the test harness scans over sources and copies the files listed there to a temporary directory, which acts as the source directory for the test compilation.

[nektro]

i think a rework of the implementation of TestContext could also achieve this

[andrewrk]

Superseded by #11288 (cc @mitchellh)