refactor: move 'gettime_i64()' to tests_common.h

Relocate the clock time getter to tests_common.h to
make it easily reusable across test programs. This
will be useful for the upcoming unit test framework.

Context - why not placing it inside testutil.h?:
The bench program links against the production-compiled library,
not its own compiled version. Therefore, `gettime_i64()` cannot
be moved to testutil.h, because testutil.h calls
`secp256k1_pubkey_save()`, which exists only in the internal
secp256k1.c and not in the public API.

Author: furszy

Makefile.am

@@ -45,6 +45,7 @@ noinst_HEADERS += src/precomputed_ecmult.h
 noinst_HEADERS += src/precomputed_ecmult_gen.h
 noinst_HEADERS += src/assumptions.h
 noinst_HEADERS += src/checkmem.h
+noinst_HEADERS += src/tests_common.h
 noinst_HEADERS += src/testutil.h
 noinst_HEADERS += src/util.h
 noinst_HEADERS += src/util_local_visibility.h

src/bench.h

@@ -12,27 +12,7 @@
 #include <stdio.h>
 #include <string.h>
 
-#if (defined(_MSC_VER) && _MSC_VER >= 1900)
-#  include <time.h>
-#else
-#  include <sys/time.h>
-#endif
-
-static int64_t gettime_i64(void) {
-#if (defined(_MSC_VER) && _MSC_VER >= 1900)
-    /* C11 way to get wallclock time */
-    struct timespec tv;
-    if (!timespec_get(&tv, TIME_UTC)) {
-        fputs("timespec_get failed!", stderr);
-        exit(EXIT_FAILURE);
-    }
-    return (int64_t)tv.tv_nsec / 1000 + (int64_t)tv.tv_sec * 1000000LL;
-#else
-    struct timeval tv;
-    gettimeofday(&tv, NULL);
-    return (int64_t)tv.tv_usec + (int64_t)tv.tv_sec * 1000000LL;
-#endif
-}
+#include "tests_common.h"
 
 #define FP_EXP (6)
 #define FP_MULT (1000000LL)

src/tests_common.h

@@ -0,0 +1,42 @@
+/***********************************************************************
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_TESTS_COMMON_H
+#define SECP256K1_TESTS_COMMON_H
+
+/***********************************************************************
+ * Test Support Utilities
+ *
+ * This file provides general-purpose functions for tests and benchmark
+ * programs. Unlike testutil.h, this file is not linked to the library,
+ * allowing each program to choose whether to run against the production
+ * API or access library internals directly.
+ ***********************************************************************/
+
+#include <stdint.h>
+
+#if (defined(_MSC_VER) && _MSC_VER >= 1900)
+#  include <time.h>
+#else
+#  include <sys/time.h>
+#endif
+
+static int64_t gettime_i64(void) {
+#if (defined(_MSC_VER) && _MSC_VER >= 1900)
+    /* C11 way to get wallclock time */
+    struct timespec tv;
+    if (!timespec_get(&tv, TIME_UTC)) {
+        fputs("timespec_get failed!", stderr);
+        exit(EXIT_FAILURE);
+    }
+    return (int64_t)tv.tv_nsec / 1000 + (int64_t)tv.tv_sec * 1000000LL;
+#else
+    struct timeval tv;
+    gettimeofday(&tv, NULL);
+    return (int64_t)tv.tv_usec + (int64_t)tv.tv_sec * 1000000LL;
+#endif
+}
+
+#endif /* SECP256K1_TESTS_COMMON_H */