changes after review: updated comments and did other small improvements

ChangeLog

+1537.	[func]		godfryd
+	Improved handling unix sockets in unit tests. Now by default
+	they are created in temporary folder under /tmp folder. This
+	fixes the issue with creating sockets with too long path
+	in the case when source folder is deeply nested.
+	(Gitlab #357,!258, git a45e2f68d7d1848adb0cf755954a3d76c9dff338)
+
 1536.	[build]		tomek
 	Many changes in keactrl, kea-admin, cql_config and sysrepo_config
 	scripts. ISC is now using shellcheck to verify portability of

doc/devel/unit-tests.dox

@@ -40,10 +40,9 @@ The following environment variable can affect the unit tests:
 - KEA_SOCKET_TEST_DIR - if set, it specifies the directory where Unix
   sockets are created. There is an operating system limitation on how
   long a Unix socket path can be, typically slightly over 100
-  characters. If you happen to build and run unit-tests in deeply nested
-  directories, this may become a problem. KEA_SOCKET_TEST_DIR can be
-  specified to instruct unit-test to use a different directory. It must
-  not end with slash.
+  characters. By default unit-tests create sockets in temporary folder
+  under /tmp folder. KEA_SOCKET_TEST_DIR can be specified to instruct
+  unit-tests to use a different directory. It must not end with slash.
 
 @section unitTestsDatabaseConfig Databases Configuration for Unit Tests
 
@@ -265,5 +264,5 @@ local   all             postgres                                trust
  [cqlsh 5.0.1 | Cassandra 3.11.1 | CQL spec 3.4.4 | Native protocol v4]
  Use HELP for help.
  cqlsh> @endverbatim\n
- 
+
  */

src/lib/testutils/sandbox.h

@@ -11,8 +11,9 @@
 
 #include <iostream>
 #include <string>
-#include <stdlib.h>
-#include <stdio.h>
+#include <cstdlib>
+#include <cstdio>
+#include <unistd.h>
 #include <ftw.h>
 
 namespace isc {
@@ -25,14 +26,12 @@ namespace test {
 /// in unit test setup phase, and then it is deleted with its content
 /// in destructor ie. in unit test tear down phase.
 class Sandbox {
+private:
     /// Path to temporary folder
     std::string path_;
 
     /// @brief Method for deleting files and folders, used in nftw traversal function.
-    static int rmFile(const char *fpath, const struct stat *sb, int typeflag, struct FTW *ftwbuf) {
-        (void)sb;
-        (void)typeflag;
-        (void)ftwbuf;
+    static int rmFile(const char *fpath, const struct stat *, int , struct FTW *) {
         return(remove(fpath));
     }
 
@@ -51,9 +50,11 @@ public:
         }
     }
 
-    // @brief Join sandbox path with indicated file subpath.
+    /// @brief Join sandbox path with indicated file subpath.
+    ///
+    /// @param file A path to file that should be joined to base path of sandbox.
     std::string join(std::string file) {
-        return path_ + "/" + file;
+        return(path_ + "/" + file);
     }
 };