test_mx_util: Avoid -Wparentheses warnings

Add extra parentheses around assigments evaluated as conditions as
suggested by -Wparentheses.

This is to avoid

    CC test_mx_util.o
    test_mx_util.c: In function ‘test_mx_strskipwhitespaces’:
    test_mx_util.c:17:4: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
	assert(s = mx_strskipwhitespaces("     abc   "));
	^
    test_mx_util.c:20:4: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
	assert(s = mx_strskipwhitespaces("abc   "));
	^
    test_mx_util.c:23:4: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
	assert(s = mx_strskipwhitespaces(""));
	^
    test_mx_util.c: In function ‘test_mx_strscan’:
    test_mx_util.c:311:5: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
	 assert(s = strdup("123 456 -789 246 abc"));
	 ^
etc.

test_mx_util.c

@@ -14,13 +14,13 @@ static void test_mx_strskipwhitespaces(void)
 {
    char *s;
 
-   assert(s = mx_strskipwhitespaces("     abc   "));
+   assert((s = mx_strskipwhitespaces("     abc   ")));
    assert(strcmp(s, "abc   ") == 0);
 
-   assert(s = mx_strskipwhitespaces("abc   "));
+   assert((s = mx_strskipwhitespaces("abc   ")));
    assert(strcmp(s, "abc   ") == 0);
 
-   assert(s = mx_strskipwhitespaces(""));
+   assert((s = mx_strskipwhitespaces("")));
    assert(strcmp(s, "") == 0);
 }
 
@@ -308,7 +308,7 @@ static void test_mx_strscan(void)
     _mx_cleanup_free_ char *line = NULL;
     _mx_cleanup_free_ struct mx_proc_pid_stat *pps = NULL;
 
-    assert(s = strdup("123 456 -789 246 abc"));
+    assert((s = strdup("123 456 -789 246 abc")));
     str = s;
 
     assert(mx_strscan_ull(&str, &ull) == 0);
@@ -333,7 +333,7 @@ static void test_mx_strscan(void)
     assert(mx_streq(s, "123 456 -789 246 abc"));
     mx_free_null(s);
 
-    assert(s = strdup("123"));
+    assert((s = strdup("123")));
     str = s;
     assert(mx_strscan_ull(&str, &ull) == 0);
     assert(ull == 123);
@@ -381,28 +381,28 @@ static void test_mx_strvec() {
     mx_strvec_push_str(&strvec,strdup("Bla"));
     mx_strvec_push_str(&strvec,strdup("lall"));
 
-    assert(str=mx_strvec_join("XXX",strvec));
+    assert((str=mx_strvec_join("XXX",strvec)));
     assert(strcmp(str,"HalloXXXBlaXXXlall")==0);
     free(str);
-    assert(str=mx_strvec_join("",strvec));
+    assert((str=mx_strvec_join("",strvec)));
     assert(strcmp(str,"HalloBlalall")==0);
     free(str);
     mx_strvec_free(strvec);
 
     strvec=mx_strvec_new();
-    assert(str=mx_strvec_join("XXX",strvec));
+    assert((str=mx_strvec_join("XXX",strvec)));
     assert(strcmp(str,"")==0);
     free(str);
     mx_strvec_push_str(&strvec,strdup("A"));
-    assert(str=mx_strvec_join("x",strvec));
+    assert((str=mx_strvec_join("x",strvec)));
     assert(strcmp(str,"A")==0);
     free(str);
     mx_strvec_push_str(&strvec,strdup(""));
-    assert(str=mx_strvec_join("x",strvec));
+    assert((str=mx_strvec_join("x",strvec)));
     assert(strcmp(str,"Ax")==0);
     free(str);
     mx_strvec_push_str(&strvec,strdup("B"));
-    assert(str=mx_strvec_join("x",strvec));
+    assert((str=mx_strvec_join("x",strvec)));
     assert(strcmp(str,"AxxB")==0);
     free(str);
     mx_strvec_free(strvec);
@@ -412,18 +412,18 @@ static void test_mx_strcat() {
     char *str;
     char *str2;
 
-    assert(str = mx_strconcat(NULL));
+    assert((str = mx_strconcat(NULL)));
     assert(mx_streq(str, ""));
     mx_free_null(str);
 
-    assert(str = mx_strconcat("abc", "123"));
+    assert((str = mx_strconcat("abc", "123")));
     assert(mx_streq(str, "abc123"));
 
-    assert(str2 = mx_strconcat(str, "def", str, "456"));
+    assert((str2 = mx_strconcat(str, "def", str, "456")));
     assert(mx_streq(str2, "abc123defabc123456"));
     mx_free_null(str2);
 
-    assert(str2 = mx_strconcat(str));
+    assert((str2 = mx_strconcat(str)));
     assert(mx_streq(str, str2));
     mx_free_null(str);
     mx_free_null(str2);