test-cases: fix race with zig run on C backend tests

Also avoid redundantly doing compile-error checks on multiple targets
for test cases where that is not helpful.

src/test.zig

@@ -1808,6 +1808,11 @@ pub const TestContext = struct {
                                 // We wouldn't be able to run the compiled C code.
                                 return; // Pass test.
                             }
+                            // Use an absolute path here so that the unique directory name
+                            // for this Case makes it into the cache hash, avoiding cache
+                            // collisions from multiple threads doing `zig run` at the same
+                            // time on the same test_case.c input filename.
+                            const abs_exe_path = try tmp.dir.realpathAlloc(arena, bin_name);
                             try argv.appendSlice(&[_][]const u8{
                                 std.testing.zig_exe_path,
                                 "run",
@@ -1818,7 +1823,7 @@ pub const TestContext = struct {
                                 "-Wno-incompatible-library-redeclaration", // https://github.com/ziglang/zig/issues/875
                                 "--",
                                 "-lc",
-                                exe_path,
+                                abs_exe_path,
                             });
                         } else switch (host.getExternalExecutor(target_info, .{ .link_libc = case.link_libc })) {
                             .native => try argv.append(exe_path),

test/cases/compile_errors/dereference_anyopaque.zig

@@ -41,6 +41,7 @@ pub export fn entry() void {
 }
 
 // error
+// target=native
 // backend=llvm
 //
 // :11:22: error: comparison of 'void' with null

test/cases/compile_errors/invalid_array_elem_ty.zig

@@ -6,6 +6,7 @@ pub export fn entry() void {
 }
 
 // error
-// backend=stage2,llvm
+// target=native
+// backend=stage2
 //
 // :4:1: error: expected type, found fn() type

test/cases/compile_errors/invalid_store_to_comptime_field.zig

@@ -14,7 +14,8 @@ pub export fn entry1() void {
     s.a = T{ .a = 2, .b = 2 };
 }
 // error
-// backend=stage2,llvm
+// target=native
+// backend=stage2
 //
 // :6:19: error: value stored in comptime field does not match the default value of the field
 // :14:19: error: value stored in comptime field does not match the default value of the field

test/cases/compile_errors/runtime_indexing_comptime_array.zig

@@ -21,7 +21,8 @@ pub export fn entry3() void {
     _ = &test_fns[i];
 }
 // error
-// backend=stage2,llvm
+// target=native
+// backend=stage2
 //
 // :6:5: error: values of type '[2]fn() void' must be comptime known, but index value is runtime known
 // :6:5: note: use '*const fn() void' for a function pointer type

test/cases/compile_errors/stage2/comptime_unreachable.zig

@@ -3,5 +3,7 @@ pub export fn entry() void {
 }
 
 // error
+// target=native
+// backend=stage2
 //
 // :2:14: error: reached unreachable code