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-undefined-builtin-result.yaml

@@ -1,12 +1,13 @@
+---
 cases:
-- data:
-  modules:
-  - |
-    package test
-    foo = units.parse_bytes("1KB")
-    bar = units.parse_bytes("foo") # undefined
-  note: functionerrors/undefined builtin result
-  query: data.test = x
-  want_result:
-  - x:
-      foo: 1000
+  - data:
+    modules:
+      - |
+        package test
+        foo = units.parse_bytes("1KB")
+        bar = units.parse_bytes("foo") # undefined
+    note: functionerrors/undefined builtin result
+    query: data.test = x
+    want_result:
+      - x:
+          foo: 1000