standalone posix tests: add skeleton

Add build.zig, README and empty test files.

test/standalone/build.zig.zon

@@ -211,6 +211,9 @@
         .tsan = .{
             .path = "tsan",
         },
+        .posix = .{
+            .path = "posix",
+        },
     },
     .paths = .{
         "build.zig",

test/standalone/posix/README.md

@@ -0,0 +1,8 @@
+## Zig standalone POSIX tests
+
+This directory is just for std.posix-related test cases that depend on
+process-wide state like the current-working directory, signal handlers,
+fork, the main thread, environment variables, etc.  Most tests (e.g,
+around file descriptors, etc) are with the unit tests in
+`lib/std/posix/test.zig`.  New tests should be with the unit tests, unless
+there is a specific reason they cannot.

test/standalone/posix/build.zig

@@ -0,0 +1,72 @@
+const std = @import("std");
+const builtin = @import("builtin");
+
+const Case = struct {
+    src_path: []const u8,
+};
+
+const cases = [_]Case{
+    .{
+        .src_path = "cwd.zig",
+    },
+    .{
+        .src_path = "getenv.zig",
+    },
+    .{
+        .src_path = "sigaction.zig",
+    },
+    .{
+        .src_path = "relpaths.zig",
+    },
+};
+
+pub fn build(b: *std.Build) void {
+    const test_step = b.step("test", "Run POSIX standalone test cases");
+    b.default_step = test_step;
+
+    const optimize = b.standardOptimizeOption(.{});
+
+    const default_target = b.resolveTargetQuery(.{});
+
+    // Run each test case built against libc-less, glibc, and musl.
+    for (cases) |case| {
+        const run_def = run_exe(b, optimize, &case, default_target, false);
+        test_step.dependOn(&run_def.step);
+
+        if (default_target.result.os.tag == .linux) {
+            const gnu_target = b.resolveTargetQuery(.{ .abi = .gnu });
+            const musl_target = b.resolveTargetQuery(.{ .abi = .musl });
+
+            const run_gnu = run_exe(b, optimize, &case, gnu_target, true);
+            const run_musl = run_exe(b, optimize, &case, musl_target, true);
+
+            test_step.dependOn(&run_gnu.step);
+            test_step.dependOn(&run_musl.step);
+        } else {
+            const run_libc = run_exe(b, optimize, &case, default_target, true);
+            test_step.dependOn(&run_libc.step);
+        }
+    }
+}
+
+fn run_exe(b: *std.Build, optimize: std.builtin.OptimizeMode, case: *const Case, target: std.Build.ResolvedTarget, link_libc: bool) *std.Build.Step.Run {
+    const exe_name = b.fmt("test-posix-{s}{s}{s}", .{
+        std.fs.path.stem(case.src_path),
+        if (link_libc) "-libc" else "",
+        if (link_libc and target.result.isGnuLibC()) "-gnu" else if (link_libc and target.result.isMuslLibC()) "-musl" else "",
+    });
+
+    const exe = b.addExecutable(.{
+        .name = exe_name,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path(case.src_path),
+            .link_libc = link_libc,
+            .optimize = optimize,
+            .target = target,
+        }),
+    });
+
+    const run_cmd = b.addRunArtifact(exe);
+
+    return run_cmd;
+}

test/standalone/posix/cwd.zig

@@ -0,0 +1 @@
+pub fn main() !void {}

test/standalone/posix/getenv.zig

@@ -0,0 +1 @@
+pub fn main() !void {}

test/standalone/posix/relpaths.zig

@@ -0,0 +1 @@
+pub fn main() !void {}

test/standalone/posix/sigaction.zig

@@ -0,0 +1 @@
+pub fn main() !void {}