tests: Even more work on error handling

realpath: Fixing undefined behaviour in the second test. If the call fails the resolved_name argument cannot be used for error checking because its state is undefined by SUSv2.
pipe: Changing the order of close and write error handling code. Errors in close could overwrite errno after write errors, returning incorrect error messages.
gmtime: Removed duplicate checks
Other fixes for fseek, rename, mktime, putwchar

tests/stdlib/realpath.c

@@ -7,25 +7,14 @@
 #include "test_helpers.h"
 
 int main(void) {
-    char* path = realpath("stdlib/realpath.c", NULL);
-    if (!path) {
-        perror("realpath");
-        exit(EXIT_FAILURE);
-    }
-    puts(path);
+    char *path_res = realpath("stdlib/realpath.c", NULL);
+    ERROR_IF(realpath, path_res, == NULL);
+    puts(path_res);
+    free(path_res);
 
-    free(path);
-
-    path = malloc(PATH_MAX);
-    memset(path, 0, PATH_MAX);
-
-    realpath("stdlib/realpath.c", path);
-    if (!path) {
-        perror("realpath");
-        free(path);
-        exit(EXIT_FAILURE);
-    }
-    puts(path);
-
-    free(path);
+    char path_arg[PATH_MAX] = { 0 };
+    char *res = realpath("stdlib/realpath.c", path_arg);
+    ERROR_IF(realpath, res, == NULL);
+    puts(path_arg);
+    free(path_arg);
 }