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-1012.yaml

@@ -1,32 +1,33 @@
+---
 cases:
-- data:
-  modules:
-  - |
-    package test1
+  - data:
+    modules:
+      - |
+        package test1
 
-    p(a) = y {
-      y = a[_]
-    }
+        p(a) = y {
+          y = a[_]
+        }
 
-    r = y {
-      data.test1.p([1, 2, 3], y)
-    }
-  note: functionerrors/function output conflict single
-  query: data.test1.r = x
-  want_error: functions must not produce multiple outputs for same inputs
-  want_error_code: eval_conflict_error
-- data:
-  modules:
-  - |
-    package test
+        r = y {
+          data.test1.p([1, 2, 3], y)
+        }
+    note: functionerrors/function output conflict single
+    query: data.test1.r = x
+    want_error: functions must not produce multiple outputs for same inputs
+    want_error_code: eval_conflict_error
+  - data:
+    modules:
+      - |
+        package test
 
-    f(_) = true
-    f(_) = false
+        f(_) = true
+        f(_) = false
 
-    r {
-      data.test.f(1)
-    }
-  note: functionerrors/function output conflict, used as boolean
-  query: data.test.r = x
-  want_error: functions must not produce multiple outputs for same inputs
-  want_error_code: eval_conflict_error
+        r {
+          data.test.f(1)
+        }
+    note: functionerrors/function output conflict, used as boolean
+    query: data.test.r = x
+    want_error: functions must not produce multiple outputs for same inputs
+    want_error_code: eval_conflict_error