Tests: Build automatically, fix compilation errors

Author: https://github.com/BenWiederhake
Commit: 29eceebdbf
Pull-request: https://github.com/SerenityOS/serenity/pull/2947

Userland/Tests/LibC/exec-should-not-search-current-directory.cpp

@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+int main()
+{
+    int fd = open("hax", O_CREAT | O_RDWR, 0755);
+    ftruncate(fd, 0);
+    close(fd);
+
+    int rc = execlp("hax", "hax", nullptr);
+    int saved_errno = errno;
+    unlink("hax");
+    if (rc == -1 && saved_errno == ENOEXEC) {
+        printf("FAIL\n");
+        return 1;
+    }
+    printf("PASS\n");
+    return 0;
+}