Add support to override b2 exec for testing.

Adds use of `B2` env var to specify the b2 exec to use for testing.
This allows us to use debug and sanitizer builds for testing.

test/BoostBuild.py

@@ -1,6 +1,6 @@
 # Copyright 2002-2005 Vladimir Prus.
 # Copyright 2002-2003 Dave Abrahams.
-# Copyright 2006 Rene Rivera.
+# Copyright 2006 Rene Ferdinand Rivera Morell.
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at
 # http://www.boost.org/LICENSE_1_0.txt)
@@ -236,12 +236,17 @@ class Tester(TestCmd.TestCmd):
                                     system output like the --verbose command
                                     line option does.
     """
-    def __init__(self, arguments=None, executable="b2",
+    def __init__(self, arguments=None, executable=None,
         match=TestCmd.match_exact, boost_build_path=None,
         translate_suffixes=True, pass_toolset=True, use_test_config=True,
         ignore_toolset_requirements=False, workdir="", pass_d0=False,
         **keywords):
 
+        if not executable:
+            executable = os.getenv('B2')
+        if not executable:
+            executable = 'b2'
+
         assert arguments.__class__ is not str
         self.original_workdir = os.path.dirname(__file__)
         if workdir and not os.path.isabs(workdir):