Refactor and add TEST_SKIP() macro.

We also rename cmdline option --skip to --exclude, as using the option
actually exclude the test from the report completely, whereas the macro
TEST_SKIP() makes it explicitly marked as SKIPPED.

(Though the --skip is silently accepted as a synonym of --exclude for
compatibility reasons.)

Closes mity#66.

README.md

@@ -369,9 +369,9 @@ tests. It can also run only subset of the unit tests as specified on the
 command line:
 
 ```sh
-$ ./test_example                # Runs all tests in the suite
-$ ./test_example test1 test2    # Runs only tests specified
-$ ./test_example --skip test3   # Runs all tests but those specified
+$ ./test_example                  # Runs all tests in the suite
+$ ./test_example test1 test2      # Runs only tests specified
+$ ./test_example --exclude test3  # Runs all tests but those specified
 ```
 
 Note that a single command line argument can select a whole group of test units
@@ -394,8 +394,8 @@ matching at least one test unit is used):
    selected.
 
 By adopting an appropriate test naming strategy, this allows user to run (or
-to skip if `--skip` is used) easily whole family of related tests with a single
-command line argument.
+to skip if `--exclude` is used) easily whole family of related tests with a
+single command line argument.
 
 For example consider test suite `test_example` which implements tests `foo-1`,
 `foo-2`, `foomatic`, `bar-1` and `bar-10`:

examples/c-example.c

@@ -42,6 +42,20 @@ test_fail(void)
     TEST_MSG("b: %d", b);
 }
 
+void
+test_skip(void)
+{
+    TEST_SKIP("Decided to skip.");
+    TEST_CHECK(1 == 0);  /* ignored check. */
+}
+
+void
+test_bad_skip(void)
+{
+    TEST_CHECK(1 == 1);
+    TEST_SKIP("Decided to skip.");      /* Cannot skip after any TEST_CHECK */
+}
+
 static void
 helper(void)
 {
@@ -75,6 +89,8 @@ test_crash(void)
 TEST_LIST = {
     { "tutorial", test_tutorial },
     { "fail",     test_fail },
+    { "skip",     test_skip },
+    { "bad-skip", test_bad_skip },
     { "abort",    test_abort },
     { "crash",    test_crash },
     { NULL, NULL }