tests: check for bzr support by importing bzrlib

This is better than starting bzr --version, because it works correctly when
running tests with a non-default Python (version).

tests/hghave

@@ -25,7 +25,11 b' def has_baz():'
     return matchoutput('baz --version 2>&1', r'baz Bazaar version')
 
 def has_bzr():
-    return matchoutput('bzr --version 2>&1', r'Bazaar \(bzr\)')
+    try:
+        import bzrlib
+        return True
+    except ImportError:
+        return False
 
 def has_cvs():
     re = r'Concurrent Versions System.*?server'