linters+testdata: Reformat all yaml testcases for linting. (#6511)

This commit adds a config for yamllint, mass-reformats all of
the existing Yaml testcases to pass linting, and adds a Yaml
linting job to the pull-request Github Actions workflow. A few 
careful exceptions and ignores were added to the linter's
config to allow keeping our existing Yaml files with minimal
reformatting.

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>

test/cases/testdata/functionerrors/test-functionerrors-1014.yaml

@@ -1,21 +1,22 @@
+---
 cases:
-- data:
-  modules:
-  - |
-    package test3
+  - data:
+    modules:
+      - |
+        package test3
 
-    p(1, a) = y {
-      y = a
-    }
+        p(1, a) = y {
+          y = a
+        }
 
-    p(x, y) = z {
-      z = x
-    }
+        p(x, y) = z {
+          z = x
+        }
 
-    r = y {
-      data.test3.p(1, 0, y)
-    }
-  note: functionerrors/function output conflict multiple
-  query: data.test3.r = x
-  want_error: functions must not produce multiple outputs for same inputs
-  want_error_code: eval_conflict_error
+        r = y {
+          data.test3.p(1, 0, y)
+        }
+    note: functionerrors/function output conflict multiple
+    query: data.test3.r = x
+    want_error: functions must not produce multiple outputs for same inputs
+    want_error_code: eval_conflict_error