test: migrate test-load-fragment to use ASSERT_* macros

Replace all assert_se() calls with appropriate ASSERT_* macros from tests.h:
- assert_se(r >= 0) → ASSERT_OK(r)
- assert_se(r == 0) → ASSERT_OK_ZERO(r)
- assert_se(r == -ERR) → ASSERT_ERROR(r, ERR)
- assert_se(expr == value) → ASSERT_EQ(expr, value)
- assert_se(str1, str2) → ASSERT_STREQ(str1, str2)
- assert_se(ptr) → ASSERT_NOT_NULL(ptr)
- assert_se(!ptr) → ASSERT_NULL(ptr)
- assert_se(expr) → ASSERT_TRUE(expr)
- assert_se(!expr) → ASSERT_FALSE(expr)

Also added unsigned suffixes (U) to numeric literals in comparisons with
unsigned types (size_t, rlim_t) to eliminate signedness warnings.

This code was written with the help of AI.

src/fundamental/string-util-fundamental.h

@@ -55,6 +55,13 @@ static inline int strcmp_ptr(const sd_char *a, const sd_char *b) {
         return CMP(a, b);
 }
 
+static inline int strncmp_ptr(const sd_char *a, const sd_char *b, size_t n) {
+        if (a && b)
+                return strncmp(a, b, n);
+
+        return CMP(a, b);
+}
+
 static inline int strcasecmp_ptr(const sd_char *a, const sd_char *b) {
         if (a && b)
                 return strcasecmp(a, b);
@@ -66,6 +73,10 @@ static inline bool streq_ptr(const sd_char *a, const sd_char *b) {
         return strcmp_ptr(a, b) == 0;
 }
 
+static inline bool strneq_ptr(const sd_char *a, const sd_char *b, size_t n) {
+        return strncmp_ptr(a, b, n) == 0;
+}
+
 static inline bool strcaseeq_ptr(const sd_char *a, const sd_char *b) {
         return strcasecmp_ptr(a, b) == 0;
 }

src/shared/tests.h

@@ -390,6 +390,19 @@ _noreturn_ void log_test_failed_internal(const char *file, int line, const char
         })
 #endif
 
+#ifdef __COVERITY__
+#  define ASSERT_STRNEQ(expr1, expr2, n) __coverity_check__(strneq_ptr((expr1), (expr2), (n)))
+#else
+#  define ASSERT_STRNEQ(expr1, expr2, n)                                                                        \
+        ({                                                                                                      \
+                const char *_expr1 = (expr1), *_expr2 = (expr2);                                                \
+                size_t _n = (n);                                                                                \
+                if (!strneq_ptr(_expr1, _expr2, _n))                                                            \
+                        log_test_failed("Expected \"%s == %s\", got \"%s != %s\" (first %zu characters)",       \
+                                        #expr1, #expr2, strnull(_expr1), strnull(_expr2), _n);                  \
+        })
+#endif
+
 #ifdef __COVERITY__
 #  define ASSERT_PTR_EQ(expr1, expr2) __coverity_check__((expr1) == (expr2))
 #else