testing: ability to read environment variables from unit tests

lib/compiler/test_runner.zig

@@ -129,6 +129,7 @@ fn mainServer(init: std.process.Init.Minimal) !void {
             },
 
             .run_test => {
+                testing.environ = init.environ;
                 testing.allocator_instance = .{};
                 testing.io_instance = .init(testing.allocator, .{
                     .argv0 = .init(init.args),
@@ -244,6 +245,7 @@ fn mainTerminal(init: std.process.Init.Minimal) void {
             if (testing.allocator_instance.deinit() == .leak) leaks += 1;
         }
         testing.log_level = .warn;
+        testing.environ = init.environ;
 
         const test_node = root_node.start(test_fn.name, 0);
         if (!have_tty) {

lib/std/testing.zig

@@ -2,6 +2,7 @@ const builtin = @import("builtin");
 
 const std = @import("std.zig");
 const Io = std.Io;
+const Environ = std.process.Environ;
 const assert = std.debug.assert;
 const math = std.math;
 
@@ -33,6 +34,8 @@ pub var allocator_instance: std.heap.GeneralPurposeAllocator(.{
 pub var io_instance: Io.Threaded = undefined;
 pub const io = if (builtin.is_test) io_instance.io() else @compileError("not testing");
 
+pub var environ: Environ = if (builtin.is_test) undefined else @compileError("not testing");
+
 /// TODO https://github.com/ziglang/zig/issues/5738
 pub var log_level = std.log.Level.warn;