tests: add helper function to autodetect CI environments

Sadly there is no standarized way to check if we're running in some
CI environment. So let's try to gather the heuristics in one helper function.

src/basic/string-util.h

@@ -33,6 +33,12 @@ static inline bool streq_ptr(const char *a, const char *b) {
         return strcmp_ptr(a, b) == 0;
 }
 
+static inline char* strstr_ptr(const char *haystack, const char *needle) {
+        if (!haystack || !needle)
+                return NULL;
+        return strstr(haystack, needle);
+}
+
 static inline const char* strempty(const char *s) {
         return s ?: "";
 }