Allow 'tests' to use StringArray, too

This allows usage i.e.:

    skiplist:
      - reason: Some test reason
        url: https://bugs.linaro.org/show_bug.cgi?id=3145
        environments: production
        boards: x15
        branches: "4.4"
        tests: run_vmtests

Before this change, environments/boards/branches could be either a list or a
value, but not tests. Fix this inconsistency. I have no idea why I didn't
anticipate this usecase.

Signed-off-by: Dan Rue <dan.rue@linaro.org>

skipgen.go

@@ -66,7 +66,7 @@ type Skipfile struct {
 		Environments StringArray
 		Boards StringArray
 		Branches StringArray
-		Tests []string
+		Tests StringArray
 	}
 }
 

skipgen_test.go

@@ -109,14 +109,10 @@ func TestSkipMinimum(t *testing.T) {
 skiplist:
   - reason: Some test reason
     url: https://bugs.linaro.org/show_bug.cgi?id=3145
-    environments:
-      - production
-    boards:
-      - x15
-    branches:
-      - "4.4"
-    tests:
-      - run_vmtests
+    environments: production # Test UnmarshallYAML
+    boards: x15 # Test UnmarshallYAML
+    branches: "4.4" # Test UnmarshallYAML
+    tests: run_vmtests # Test UnmarshallYAML
 `
 	skips, err := parseSkipfile([]byte(skipAll))
 	if err != nil {