Detect when a fasttext executable is available in PATH (#3264)

Also check that fasttext exists in FT_HOME and is executable.

This is useful when using a distro like Debian that
has a package of fasttext available to install.

Co-authored-by: Michael Penkov <m@penkov.dev>

gensim/test/test_fasttext.py

@@ -8,6 +8,7 @@
 import logging
 import unittest
 import os
+import shutil
 import subprocess
 import struct
 import sys
@@ -44,7 +45,8 @@
 BUCKET = 10000
 
 FT_HOME = os.environ.get("FT_HOME")
-FT_CMD = os.path.join(FT_HOME, "fasttext") if FT_HOME else None
+FT_CMD = shutil.which("fasttext", path=FT_HOME) or \
+         shutil.which("fasttext")
 
 
 new_sentences = [
@@ -1661,7 +1663,7 @@ def _save_test_model(out_base_fname, model_params):
     subprocess.check_call(cmd)
 
 
-@unittest.skipIf(not FT_HOME, "FT_HOME env variable not set, skipping test")
+@unittest.skipIf(not FT_CMD, "fasttext not in FT_HOME or PATH, skipping test")
 class SaveFacebookByteIdentityTest(unittest.TestCase):
     """
     This class containts tests that check the following scenario:
@@ -1708,7 +1710,7 @@ def line_to_array(line):
     return np.array([line_to_array(line) for line in out.splitlines()], dtype=np.float32)
 
 
-@unittest.skipIf(not os.environ.get("FT_HOME", None), "FT_HOME env variable not set, skipping test")
+@unittest.skipIf(not FT_CMD, "fasttext not in FT_HOME or PATH, skipping test")
 class SaveFacebookFormatReadingTest(unittest.TestCase):
     """
     This class containts tests that check the following scenario: