Intent of valid_string, valid_string_escape_char

All 3 exact tests in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 3 tests
- server/ 3 tests
  - common/ 3 tests
    
    persistence/ 3 tests
    
    visibility/ 3 tests
    
    store/ 3 tests
    
    sql/ 3 tests
    
    TestPostgreSQLQueryConverterSuite/ 3 tests
    
    TestConvertValueExpr/ 3 tests
    
    go.temporal.io/server/common/persistence/visibility/store/sql/TestPostgreSQLQueryConverterSuite/TestConvertValueExpr/valid_string Introduced here
    
    go.temporal.io/server/common/persistence/visibility/store/sql/TestPostgreSQLQueryConverterSuite/TestConvertValueExpr/valid_string_escape_char Introduced here
    
    go.temporal.io/server/common/persistence/visibility/store/sql/TestPostgreSQLQueryConverterSuite/TestConvertValueExpr/valid_tuple Inherited valid_tuple