misc/cgo/testsanitizers: move test source files into testdata directory

If we run 'go test ./...' in the misc module, we don't want to see
errors for these standalone files.

We could instead add +ignore tags to each file individually, but this
is exactly what a testdata directory is for.

Updates #30228

Change-Id: I7047ad888dd6aff701f5982d58b6a79f6a487c58
Reviewed-on: https://go-review.googlesource.com/c/163417
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

misc/cgo/testsanitizers/cc_test.go

@@ -394,7 +394,7 @@ func (c *config) checkRuntime() (skip bool, err error) {
 
 // srcPath returns the path to the given file relative to this test's source tree.
 func srcPath(path string) string {
-	return filepath.Join("src", path)
+	return filepath.Join("testdata", path)
 }
 
 // A tempDir manages a temporary directory within a test.