Follow-up for Fixing SSIM Test Method Names (#8750)

This fixes some other issues which were described in #8746 .

### Description
This is a follow-up for #8746, thanks and credits to @ericspod! 👍 

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Benedikt Johannes <benedikt.johannes.hofer@gmail.com>
Signed-off-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: benediktjohannes

tests/metrics/test_ssim_metric.py

@@ -21,7 +21,7 @@
 
 class TestSSIMMetric(unittest.TestCase):
 
-    def test2d_gaussian(self):
+    def test_2d_gaussian(self):
         set_determinism(0)
         preds = torch.abs(torch.randn(2, 3, 16, 16))
         target = torch.abs(torch.randn(2, 3, 16, 16))
@@ -32,9 +32,9 @@ def test2d_gaussian(self):
         metric(preds, target)
         result = metric.aggregate()
         expected_value = 0.045415
-        self.assertTrue(expected_value - result.item() < 0.000001)
+        self.assertTrue(abs(expected_value - result.item()) < 0.000001)
 
-    def test2d_uniform(self):
+    def test_2d_uniform(self):
         set_determinism(0)
         preds = torch.abs(torch.randn(2, 3, 16, 16))
         target = torch.abs(torch.randn(2, 3, 16, 16))
@@ -45,9 +45,9 @@ def test2d_uniform(self):
         metric(preds, target)
         result = metric.aggregate()
         expected_value = 0.050103
-        self.assertTrue(expected_value - result.item() < 0.000001)
+        self.assertTrue(abs(expected_value - result.item()) < 0.000001)
 
-    def test3d_gaussian(self):
+    def test_3d_gaussian(self):
         set_determinism(0)
         preds = torch.abs(torch.randn(2, 3, 16, 16, 16))
         target = torch.abs(torch.randn(2, 3, 16, 16, 16))