anchors: Fix invalid YAML in aliases test cases

Although accepted by PyYAML, `{*x: 4}` is not valid YAML: it should be
noted `{*x : 4}`. The reason is that a colon can be part of an anchor
name. See this comment from Tina Müller for more details:
#550 (comment)

Even if it's not a problem for yamllint, let's fix our tests to include
valid YAML snippets.

tests/rules/test_anchors.py

@@ -46,7 +46,7 @@ def test_disabled(self):
                    '  <<: *b_m\n'
                    '  foo: bar\n'
                    '---\n'
-                   '{a: 1, &x b: 2, c: &y 3, *x: 4, e: *y}\n'
+                   '{a: 1, &x b: 2, c: &y 3, *x : 4, e: *y}\n'
                    '...\n', conf)
         self.check('---\n'
                    '- &i 42\n'
@@ -74,7 +74,7 @@ def test_disabled(self):
                    '  <<: *b_m\n'
                    '  foo: bar\n'
                    '---\n'
-                   '{a: 1, &x b: 2, c: &x 3, *x: 4, e: *y}\n'
+                   '{a: 1, &x b: 2, c: &x 3, *x : 4, e: *y}\n'
                    '...\n', conf)
 
     def test_forbid_undeclared_aliases(self):
@@ -106,7 +106,7 @@ def test_forbid_undeclared_aliases(self):
                    '  <<: *b_m\n'
                    '  foo: bar\n'
                    '---\n'
-                   '{a: 1, &x b: 2, c: &y 3, *x: 4, e: *y}\n'
+                   '{a: 1, &x b: 2, c: &y 3, *x : 4, e: *y}\n'
                    '...\n', conf)
         self.check('---\n'
                    '- &i 42\n'
@@ -134,7 +134,7 @@ def test_forbid_undeclared_aliases(self):
                    '  <<: *b_m\n'
                    '  foo: bar\n'
                    '---\n'
-                   '{a: 1, &x b: 2, c: &x 3, *x: 4, e: *y}\n'
+                   '{a: 1, &x b: 2, c: &x 3, *x : 4, e: *y}\n'
                    '...\n', conf,
                    problem1=(9, 3),
                    problem2=(10, 3),
@@ -173,7 +173,7 @@ def test_forbid_duplicated_anchors(self):
                    '  <<: *b_m\n'
                    '  foo: bar\n'
                    '---\n'
-                   '{a: 1, &x b: 2, c: &y 3, *x: 4, e: *y}\n'
+                   '{a: 1, &x b: 2, c: &y 3, *x : 4, e: *y}\n'
                    '...\n', conf)
         self.check('---\n'
                    '- &i 42\n'
@@ -201,7 +201,7 @@ def test_forbid_duplicated_anchors(self):
                    '  <<: *b_m\n'
                    '  foo: bar\n'
                    '---\n'
-                   '{a: 1, &x b: 2, c: &x 3, *x: 4, e: *y}\n'
+                   '{a: 1, &x b: 2, c: &x 3, *x : 4, e: *y}\n'
                    '...\n', conf,
                    problem1=(5, 3),
                    problem2=(6, 3),