clock-util: make clock_is_localtime() testable and add initial tests

Add path argument to clock_is_localtime() and default to "/etc/adjtime" if it's
NULL. This makes the function testable.

Add test-clock: initial test cases for some scenarios, using a temporary file.
This also checks the behaviour with a NULL (i. e. the system's /etc/adjtime)
file.

src/basic/clock-util.c

@@ -69,9 +69,12 @@ int clock_set_hwclock(const struct tm *tm) {
         return 0;
 }
 
-int clock_is_localtime(void) {
+int clock_is_localtime(const char* adjtime_path) {
         _cleanup_fclose_ FILE *f;
 
+        if (adjtime_path == NULL)
+                adjtime_path = "/etc/adjtime";
+
         /*
          * The third line of adjtime is "UTC" or "LOCAL" or nothing.
          *   # /etc/adjtime
@@ -79,7 +82,7 @@ int clock_is_localtime(void) {
          *   0
          *   UTC
          */
-        f = fopen("/etc/adjtime", "re");
+        f = fopen(adjtime_path, "re");
         if (f) {
                 char line[LINE_MAX];
                 bool b;

src/basic/clock-util.h

@@ -21,7 +21,7 @@
 
 #include <time.h>
 
-int clock_is_localtime(void);
+int clock_is_localtime(const char* adjtime_path);
 int clock_set_timezone(int *min);
 int clock_reset_timewarp(void);
 int clock_get_hwclock(struct tm *tm);