Add backups.testProviderConfig

fields like format/retention won't be validated here since it's only
testing the access credentials

src/backups.js

@@ -2,6 +2,7 @@
 
 exports = module.exports = {
     testConfig: testConfig,
+    testProviderConfig: testProviderConfig,
 
     getByStatePaged: getByStatePaged,
     getByAppIdPaged: getByAppIdPaged,
@@ -118,6 +119,17 @@ function testConfig(backupConfig, callback) {
     api(backupConfig.provider).testConfig(backupConfig, callback);
 }
 
+
+function testProviderConfig(backupConfig, callback) {
+    assert.strictEqual(typeof backupConfig, 'object');
+    assert.strictEqual(typeof callback, 'function');
+
+    var func = api(backupConfig.provider);
+    if (!func) return callback(new BoxError(BoxError.BAD_FIELD, 'unknown storage provider', { field: 'provider' }));
+
+    api(backupConfig.provider).testConfig(backupConfig, callback);
+}
+
 function getByStatePaged(state, page, perPage, callback) {
     assert.strictEqual(typeof state, 'string');
     assert(typeof page === 'number' && page > 0);