Add more paths helper methods

Add PathsForOutput to convert multiple strings into WritablePaths.

Add OutputPath.InSameDir to build a new OutputPath pointing to a
file in the same directory as an existing OutputPath.

Add WritablePathForTesting and WritablePathsForTesting that mirror
PathForTesting and PathsForTesting but return WritablePaths.

Add PathContextForTesting to return a minimal PathContext
implementation.

Test: paths_test.go
Change-Id: I9708eb164b273514a96dae0a260ef9a963fb9bcf

cc/cc_test.go

@@ -1039,7 +1039,7 @@ var splitListForSizeTestCases = []struct {
 
 func TestSplitListForSize(t *testing.T) {
 	for _, testCase := range splitListForSizeTestCases {
-		out, _ := splitListForSize(android.PathsForTesting(testCase.in), testCase.size)
+		out, _ := splitListForSize(android.PathsForTesting(testCase.in...), testCase.size)
 
 		var outStrings [][]string