test: introduce (mini) unit test framework

Lightweight unit testing framework, providing a structured way to define,
execute, and report tests. It includes a central test registry, a flexible
command-line argument parser of the form "--key=value" / "-k=value" /
"-key=value" (facilitating future framework extensions), ability to run
tests in parallel and accumulated test time logging reports.

So far the supported command-line args are:
- "--jobs=<num>" or "-j=<num>" to specify the number of parallel workers.
- "--seed=<hex>" to specify the RNG seed (random if not set).
- "--iterations=<num>" or "-i=<num>" to specify the number of iterations.

Compatibility Note:
To stay compatible with previous versions, the framework also supports
the two original positional arguments: the iterations count and the
RNG seed (in that order).

configure.ac

@@ -443,6 +443,14 @@ if test x"$enable_experimental" = x"no"; then
   fi
 fi
 
+# Check for concurrency support (tests only)
+if test "x$enable_tests" != x"no"; then
+  AC_CHECK_HEADERS([sys/types.h sys/wait.h unistd.h])
+  AS_IF([test "x$ac_cv_header_sys_types_h" = xyes && test "x$ac_cv_header_sys_wait_h" = xyes &&
+         test "x$ac_cv_header_unistd_h" = xyes], [TEST_DEFINES="-DSUPPORTS_CONCURRENCY=1"], TEST_DEFINES="")
+  AC_SUBST(TEST_DEFINES)
+fi
+
 ###
 ### Generate output
 ###