testenv: enable running tests on different wget binary

* testenv/README: Document $WGET_PATH
* testenv/test/base_test.py: Use $WGET_PATH instead ../src/wget if set

Previously tests in testenv/ directory were run only on wget binary
which was built from sources in src/ directory. However as a
wget maintainer in a Linux distribution, I would like to be able to run
upstream tests on the wget binary distributed with the distribution.

This change enables one to define WGET_PATH environment variable
to a path to wget binary which should be used by tests.

Signed-off-by: Tomas Hozza <thozza@redhat.com>

testenv/README

@@ -99,6 +99,10 @@ Environment Variables:
   If it is set to "1", valgrind memcheck is enabled with hard coded options.
   This variable is set by ./configure --enable-valgrind-tests.
 * SSL_TESTS: This must be set to run any https tests.
+* WGET_PATH: Set this environment variable to a path to wget binary on which you
+  want to run tests. This is useful for OS distributions, which want to reuse
+  upstream tests for testing wget build that they distribute. If the variable is
+  not set, the "../src/wget" binary is used by tests.
 
 
 File Structure:

testenv/test/base_test.py

@@ -111,8 +111,12 @@ class BaseTest:
 
     def gen_cmd_line(self):
         test_path = os.path.abspath(".")
-        wget_path = os.path.abspath(os.path.join(test_path,
-                                                 "..", '..', 'src', "wget"))
+        if os.getenv("WGET_PATH"):
+            wget_path = os.path.abspath(os.getenv("WGET_PATH"))
+        else:
+            wget_path = os.path.abspath(os.path.join(test_path,
+                                                     "..", '..', 'src',
+                                                     "wget"))
         wget_options = '--debug --no-config %s' % self.wget_options
 
         valgrind = os.getenv("VALGRIND_TESTS", "")