[C] Update build-and-test.bash for CI errors (#4312)

* Update build-and-test.bash

add check for non autogenerated test file

* Update README.md.mustache

Add instructions on how to test petstore sample manual tests.

* remove petstore sample from readme

Author: zhemant

samples/client/petstore/c/build-and-test.bash

@@ -16,6 +16,7 @@ cmake .
 
 make 
 
-./unit-manual-PetAPI
-./unit-manual-UserAPI
-./unit-manual-StoreAPI
+if [ -f unit-manual-PetAPI ]; then ./unit-manual-PetAPI; fi
+if [ -f unit-manual-UserAPI ]; then ./unit-manual-UserAPI; fi
+if [ -f unit-manual-StoreAPI ]; then ./unit-manual-StoreAPI; fi
+