bpo-38234: test_embed: test pyvenv.cfg and pybuilddir.txt (GH-16366)

Add test_init_pybuilddir() and test_init_pyvenv_cfg() to test_embed
to test pyvenv.cfg and pybuilddir.txt configuration files.

Fix sysconfig._generate_posix_vars(): pybuilddir.txt uses UTF-8
encoding, not ASCII.

Lib/sysconfig.py

@@ -412,7 +412,7 @@ def _generate_posix_vars():
         pprint.pprint(vars, stream=f)
 
     # Create file used for sys.path fixup -- see Modules/getpath.c
-    with open('pybuilddir.txt', 'w', encoding='ascii') as f:
+    with open('pybuilddir.txt', 'w', encoding='utf8') as f:
         f.write(pybuilddir)
 
 def _init_posix(vars):