tests: utils: Add virTestLoadFilePath helper

This new helper loads and returns a file from 'abs_srcdir'. By using
variable arguments for the function, it's not necessary to format the
path separately in the test cases.

Reviewed-by: Eric Blake <eblake@redhat.com>

tests/testutils.h

@@ -52,6 +52,8 @@ int virTestRun(const char *title,
                int (*body)(const void *data),
                const void *data);
 int virTestLoadFile(const char *file, char **buf);
+char *virTestLoadFilePath(const char *p, ...)
+    ATTRIBUTE_SENTINEL;
 int virTestCaptureProgramOutput(const char *const argv[], char **buf, int maxlen);
 
 void virTestClearCommandPath(char *cmdset);