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).

Makefile.am

@@ -47,6 +47,8 @@ noinst_HEADERS += src/assumptions.h
 noinst_HEADERS += src/checkmem.h
 noinst_HEADERS += src/tests_common.h
 noinst_HEADERS += src/testutil.h
+noinst_HEADERS += src/unit_test.h
+noinst_HEADERS += src/unit_test.c
 noinst_HEADERS += src/util.h
 noinst_HEADERS += src/util_local_visibility.h
 noinst_HEADERS += src/int128.h
@@ -121,7 +123,7 @@ if USE_TESTS
 TESTS += noverify_tests
 noinst_PROGRAMS += noverify_tests
 noverify_tests_SOURCES = src/tests.c
-noverify_tests_CPPFLAGS = $(SECP_CONFIG_DEFINES)
+noverify_tests_CPPFLAGS = $(SECP_CONFIG_DEFINES) $(TEST_DEFINES)
 noverify_tests_LDADD = $(COMMON_LIB) $(PRECOMPUTED_LIB)
 noverify_tests_LDFLAGS = -static
 if !ENABLE_COVERAGE