[gcov][test] Call wait() to make gcov-fork.c reliable

[gcov][test] Call wait() to make gcov-fork.c reliable

If the parent exit before the child, the line counts might be 1.

next:18'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          26:  1: 21: if (fork() == -1) return 1; // CHECK-NEXT: 1: [[#@LINE]]:
next:18'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          27:  1: 22: func2(); // CHECK-NEXT: 2: [[#@LINE]]:
next:18'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          28:  1: 23: return 0; // CHECK-NEXT: 2: [[#@LINE]]:
next:18'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~