test: add missing header to test.h

```
../src/shared/tests.h:361:28: error: implicit declaration of function ‘fork’ [-Werror=implicit-function-declaration]
  361 |                 int _pid = fork();                                                                              \
      |                            ^~~~
../src/test/test-macro.c:1114:9: note: in expansion of macro ‘ASSERT_SIGNAL’
 1114 |         ASSERT_SIGNAL(ASSERT_OK(-1), SIGABRT);
      |         ^~~~~~~~~~~~~
../src/shared/tests.h:369:25: error: implicit declaration of function ‘_exit’ [-Werror=implicit-function-declaration]
  369 |                         _exit(EXIT_SUCCESS);                                                                    \
      |                         ^~~~~
../src/test/test-macro.c:1114:9: note: in expansion of macro ‘ASSERT_SIGNAL’
 1114 |         ASSERT_SIGNAL(ASSERT_OK(-1), SIGABRT);
      |         ^~~~~~~~~~~~~
../src/shared/tests.h:369:25: warning: incompatible implicit declaration of built-in function ‘_exit’ [-Wbuiltin-declaration-mismatch]
  369 |                         _exit(EXIT_SUCCESS);                                                                    \
      |                         ^~~~~
../src/test/test-macro.c:1114:9: note: in expansion of macro ‘ASSERT_SIGNAL’
 1114 |         ASSERT_SIGNAL(ASSERT_OK(-1), SIGABRT);
      |         ^~~~~~~~~~~~~
../src/shared/tests.h:369:25: warning: incompatible implicit declaration of built-in function ‘_exit’ [-Wbuiltin-declaration-mismatch]
  369 |                         _exit(EXIT_SUCCESS);                                                                    \
      |                         ^~~~~
../src/test/test-macro.c:1115:9: note: in expansion of macro ‘ASSERT_SIGNAL’
 1115 |         ASSERT_SIGNAL(ASSERT_OK(-ENOANO), SIGABRT);
      |         ^~~~~~~~~~~~~
../src/test/test-macro.c:1119:21: error: implicit declaration of function ‘getpid’; did you mean ‘getpt’? [-Werror=implicit-function-declaration]
 1119 |         ASSERT_TRUE(getpid());
      |                     ^~~~~~
```

Follow-up for systemd#32189

src/shared/tests.h

@@ -3,6 +3,7 @@
 
 #include <stdbool.h>
 #include <sys/prctl.h>
+#include <unistd.h>
 
 #include "sd-daemon.h"