loadNXDTestFile: use `compile` for better tracebacks on errors

When using compile with the actual file path, we can have better tracebacks
in case of errors.

before:

    Traceback (most recent call last):
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/bin/nxdtest", line 22, in <module>
        sys.exit(nxdtest.main())
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 142, in main
        tenv = loadNXDTestFile('.nxdtest')
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 75, in loadNXDTestFile
        six.exec_(src, g)
      File "<string>", line 77, in <module>
    NameError: name 'Pylint' is not defined

after:

    Traceback (most recent call last):
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/bin/nxdtest", line 22, in <module>
        sys.exit(nxdtest.main())
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 142, in main
        tenv = loadNXDTestFile('.nxdtest')
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 75, in loadNXDTestFile
        six.exec_(compile(src, os.path.realpath(path), 'exec'), g)
      File "/srv/slapgrid/slappart3/srv/runner/instance/slappart8/var/nxdtest/.nxdtest", line 77, in <module>
        summaryf=Pylint.summary,
    NameError: name 'Pylint' is not defined

/reviewed-by @kirr
/reviewed-in nexedi/nxdtest!10

nxdtest/__init__.py

@@ -73,7 +73,7 @@ def loadNXDTestFile(path): # -> TestEnv
          'UnitTest': UnitTest,}
     with open(path, "r") as f:
         src = f.read()
-    six.exec_(src, g)
+    six.exec_(compile(src, os.path.realpath(path), 'exec'), g)
     return t
 
 # TestCase defines one test case to run.

nxdtest/nxdtest_test.py

@@ -89,6 +89,18 @@ TestCase('TESTNAME', ['echo'], summaryf="error")
     assert "TypeError" in captured.err
 
 
+def test_error_execing_nxdtest_file(run_nxdtest, capsys):
+    with pytest.raises(ZeroDivisionError) as excinfo:
+        run_nxdtest(
+        """\
+1 / 0
+"""
+    )
+    assert '1 / 0' in str(excinfo.traceback[-1])
+    # The actual .nxdtest filename is also included in traceback
+    assert ".nxdtest':1" in str(excinfo.traceback[-1])
+
+
 def test_run_argument(run_nxdtest, capsys):
     run_nxdtest(
         """\