TST: test_streamplot_2D faster

* speed up `test_streamplot_2D()` because it has been
reported to take ~20 seconds on a regular basis
in CI in MDAnalysisgh-4209

* we don't really need to plot the output, which
was taking most of the time, and instead we can
just check the data structures that MDAnalysis
returns (this may be a better test by some definitions
anyway...); I suppose we could also spot check
a few values in the arrays if we wanted as well

* locally, that single test seems to run in 0.39 s
on this branch vs. 4.7 s on `develop`

testsuite/MDAnalysisTests/visualization/test_streamlines.py

@@ -51,7 +51,7 @@ def membrane_xtc(tmpdir_factory, univ):
            z_delta += 0.02
     return str(tmp_xtc)
 
-def test_streamplot_2D(membrane_xtc, univ, tmpdir):
+def test_streamplot_2D(membrane_xtc, univ):
     # simple roundtrip test to ensure that
     # a plot is generated by the 2D streamplot
     # code
@@ -67,22 +67,10 @@ def test_streamplot_2D(membrane_xtc, univ, tmpdir):
                                                         ymax=univ.atoms.positions[...,1].max(),
                                                         maximum_delta_magnitude=2.0,
                                                         num_cores=1)
-    x = np.linspace(univ.atoms.positions[...,0].min(),
-                    univ.atoms.positions[...,0].max(),
-                    5)
-    y = np.linspace(univ.atoms.positions[...,1].min(),
-                    univ.atoms.positions[...,1].max(),
-                    5)
-    speed = np.sqrt(u1*u1 + v1*v1)
-    fig = plt.figure()
-    ax = fig.add_subplot(111, aspect='equal')
-    ax.streamplot(x, y, u1, v1, density=(10,10), color=speed, linewidth=3*speed/speed.max(),
-                  cmap=plt.cm.viridis)
-    plot_outpath = os.path.join(str(tmpdir), 'testing_streamline.png')
-    fig.savefig(plot_outpath, dpi=300)
-
-    with open(plot_outpath, 'rb'):
-        pass
+    assert u1.shape == (5, 5)
+    assert v1.shape == (5, 5)
+    assert_allclose(avg, 0.965194167)
+    assert_allclose(std, 4.444808820e-06)
 
 
 def test_streamplot_2D_zero_return(membrane_xtc, univ, tmpdir):