zig test: std.testing.zig_exe_path is now available

it will be set to the path of the zig executable which is running
`zig test`.

lib/std/special/test_runner.zig

@@ -11,7 +11,15 @@ pub const io_mode: io.Mode = builtin.test_io_mode;
 
 var log_err_count: usize = 0;
 
+var args_buffer: [std.fs.MAX_PATH_BYTES + std.mem.page_size]u8 = undefined;
+var args_allocator = std.heap.FixedBufferAllocator.init(&args_buffer);
+
 pub fn main() anyerror!void {
+    const args = std.process.argsAlloc(&args_allocator.allocator) catch {
+        @panic("Too many bytes passed over the CLI to the test runner");
+    };
+    std.testing.zig_exe_path = args[1];
+
     const test_fn_list = builtin.test_functions;
     var ok_count: usize = 0;
     var skip_count: usize = 0;

lib/std/testing.zig

@@ -21,6 +21,10 @@ pub var base_allocator_instance = std.heap.FixedBufferAllocator.init("");
 /// TODO https://github.com/ziglang/zig/issues/5738
 pub var log_level = std.log.Level.warn;
 
+/// This is available to any test that wants to execute Zig in a child process.
+/// It will be the same executable that is running `zig test`.
+pub var zig_exe_path: []const u8 = undefined;
+
 /// This function is intended to be used only in tests. It prints diagnostics to stderr
 /// and then aborts when actual_error_union is not expected_error.
 pub fn expectError(expected_error: anyerror, actual_error_union: anytype) void {

src/main.zig

@@ -1828,7 +1828,9 @@ fn buildOutputType(
                     else => unreachable,
                 }
             }
-            try argv.append(exe_path);
+            try argv.appendSlice(&[_][]const u8{
+                exe_path, self_exe_path,
+            });
         } else {
             for (test_exec_args.items) |arg| {
                 try argv.append(arg orelse exe_path);