log: allow for -d log_default_level=debug, to change the default log level for programs and tests, that do not explicitly call log.set_level/1 themselves.

vlib/log/file_log_test.v

@@ -14,6 +14,7 @@ fn test_reopen() {
 
 	dump(lfolder)
 	mut l := log.new_thread_safe_log()
+	l.set_level(.debug)
 	l.set_full_logpath(lpath1)
 	l.warn('one warning')
 	l.error('one error')
@@ -42,6 +43,7 @@ fn test_reopen() {
 }
 
 fn test_set_always_flush() {
+	log.set_level(.debug)
 	lfolder := os.join_path(os.vtmp_dir(), rand.ulid())
 	lpath1 := os.join_path(lfolder, 'current.log')
 	os.mkdir_all(lfolder)!

vlib/log/safe_log.v

@@ -13,8 +13,10 @@ pub mut:
 // new_thread_safe_log returns a new log structure, whose methods are safe
 // to call by multiple threads.
 pub fn new_thread_safe_log() &ThreadSafeLog {
+	slevel := $d('log_default_level', 'info')
+	level := level_from_tag(slevel.to_upper()) or { panic('invalid log_default_level: ${slevel}') }
 	mut x := &ThreadSafeLog{
-		level: .info
+		level: level
 	}
 	return x
 }