Use a fixture for npz files in test_bat (#2981)

Fixes #2979

## Work done in this PR

The file test_bat_IO.npy was created in one test and used in
another one. This leads to the second test possibly failing
when the tests are run in parallel. This commit moves creating
the file into a fixture that the two tests depends.

The new fixture also uses the tmpdir fixture to avoid the file
created during the tests to remain in the test directory.

package/CHANGELOG

@@ -58,6 +58,8 @@ Fixes
     Universe (PR #2893)
   * ensure that unistd.h is included on macOS when compiling ENCORE's spe.c
     (Issue #2934)
+  * Fix tests for analysis.bat that could fail when run in parallel and that
+    would create a test artifact (Issue #2979, PR #2981)
 
 Enhancements
   * Refactored analysis.helanal into analysis.helix_analysis

testsuite/MDAnalysisTests/analysis/test_bat.py

@@ -45,6 +45,14 @@ def bat(self, selected_residues):
         R.run()
         return R.bat
 
+    @pytest.fixture
+    def bat_npz(self, tmpdir, selected_residues):
+        filename = str(tmpdir / 'test_bat_IO.npy')
+        R = BAT(selected_residues)
+        R.run()
+        R.save(filename)
+        return filename
+
     def test_bat_root_selection(self, selected_residues):
         R = BAT(selected_residues)
         assert_equal(R._root.indices, [8, 2, 1],
@@ -79,11 +87,8 @@ def test_bat_reconstruction(self, selected_residues, bat):
             err_msg="error: Reconstructed Cartesian coordinates " + \
                     "don't match original")
 
-    def test_bat_IO(self, selected_residues, bat):
-        R = BAT(selected_residues)
-        R.run()
-        R.save('test_bat_IO.npy')
-        R2 = BAT(selected_residues, filename='test_bat_IO.npy')
+    def test_bat_IO(self, bat_npz, selected_residues, bat):
+        R2 = BAT(selected_residues, filename=bat_npz)
         test_bat = R2.bat
         assert_almost_equal(
             bat,
@@ -107,8 +112,8 @@ def test_bat_disconnected_atom_group(self):
         with pytest.raises(ValueError):
             R = BAT(selected_residues)
 
-    def test_bat_incorrect_dims(self):
+    def test_bat_incorrect_dims(self, bat_npz):
         u = mda.Universe(PSF, DCD)
         selected_residues = u.select_atoms("resid 1-3")
         with pytest.raises(ValueError):
-            R = BAT(selected_residues, filename='test_bat_IO.npy')
+            R = BAT(selected_residues, filename=bat_npz)