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/discover_available.py

@@ -0,0 +1,47 @@
+import subprocess
+import sys
+
+from unit.check.chroot import check_chroot
+from unit.check.go import check_go
+from unit.check.isolation import check_isolation
+from unit.check.njs import check_njs
+from unit.check.node import check_node
+from unit.check.regex import check_regex
+from unit.check.tls import check_openssl
+from unit.check.unix_abstract import check_unix_abstract
+from unit.log import Log
+from unit.option import option
+
+
+def discover_available(unit):
+    output_version = subprocess.check_output(
+        [unit['unitd'], '--version'], stderr=subprocess.STDOUT
+    ).decode()
+
+    # wait for controller start
+
+    if Log.wait_for_record(r'controller started') is None:
+        Log.print_log()
+        sys.exit("controller didn't start")
+
+    # discover modules from log file
+
+    for module in Log.findall(r'module: ([a-zA-Z]+) (.*) ".*"$'):
+        versions = option.available['modules'].setdefault(module[0], [])
+        if module[1] not in versions:
+            versions.append(module[1])
+
+    # discover modules using check
+
+    option.available['modules']['go'] = check_go()
+    option.available['modules']['njs'] = check_njs(output_version)
+    option.available['modules']['node'] = check_node()
+    option.available['modules']['openssl'] = check_openssl(output_version)
+    option.available['modules']['regex'] = check_regex(output_version)
+
+    # Discover features using check. Features should be discovered after
+    # modules since some features can require modules.
+
+    option.available['features']['chroot'] = check_chroot()
+    option.available['features']['isolation'] = check_isolation()
+    option.available['features']['unix_abstract'] = check_unix_abstract()