selftests/functional/test_basic.py: make echo output safe

In the module global scope, "man echo" is executed to attempt to
determine if it's a GNU echo.  But, "man echo" in a locale such
as UTF-8 produces funky copyright characters:

  $ LANG=en_US.UTF-8 man echo | chardetect -
  <stdin>: windows-1252 with confidence 0.73

Which can not be decoded by the UTF-8 codec, producing
UnicodeDecodeErrors.  Even if that wasn't the case, it's better to
control the locale of command that can generate varied output, and
that we depend on.

  $ LANG=C man echo | chardetect -
  <stdin>: ascii with confidence 1.0
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/functional/test_basic.py

@@ -138,7 +138,9 @@ CC_BINARY = probe_binary('cc')
 GNU_ECHO_BINARY = probe_binary('echo')
 if GNU_ECHO_BINARY is not None:
     if probe_binary('man') is not None:
-        echo_manpage = process.run('man %s' % os.path.basename(GNU_ECHO_BINARY)).stdout
+        echo_cmd = 'man %s' % os.path.basename(GNU_ECHO_BINARY)
+        echo_manpage = process.run(echo_cmd, env={'LANG': 'C'},
+                                   encoding='ascii').stdout
         if b'-e' not in echo_manpage:
             GNU_ECHO_BINARY = probe_binary('gecho')
 READ_BINARY = probe_binary('read')