Tests: prerequisites checking reworked.

Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduced "require" fixture to provide per-test requirements check.

test/unit/check/check_prerequisites.py

@@ -0,0 +1,63 @@
+import pytest
+from unit.option import option
+
+
+def check_prerequisites(prerequisites):
+    if 'privileged_user' in prerequisites:
+        if prerequisites['privileged_user'] and not option.is_privileged:
+            pytest.skip(
+                'privileged user required',
+                allow_module_level=True,
+            )
+        elif not prerequisites['privileged_user'] and option.is_privileged:
+            pytest.skip(
+                'unprivileged user required',
+                allow_module_level=True,
+            )
+
+    missed = []
+
+    # check modules
+
+    if 'modules' in prerequisites:
+        available = option.available['modules']
+
+        for module in prerequisites['modules']:
+            if module in available and available[module]:
+                continue
+
+            missed.append(module)
+
+    if missed:
+        pytest.skip(
+            f'Unit has no {", ".join(missed)} module(s)',
+            allow_module_level=True,
+        )
+
+    # check features
+
+    if 'features' in prerequisites:
+        available = option.available['features']
+        require = prerequisites['features']
+
+        for feature in require:
+            avail_feature = available[feature]
+
+            if feature in available and avail_feature:
+                if isinstance(require[feature], list) and isinstance(
+                    avail_feature, dict
+                ):
+                    avail_keys = avail_feature.keys()
+
+                    for key in require[feature]:
+                        if key not in avail_keys:
+                            missed.append(f'{feature}/{key}')
+                continue
+
+            missed.append(feature)
+
+    if missed:
+        pytest.skip(
+            f'{", ".join(missed)} feature(s) not supported',
+            allow_module_level=True,
+        )