Add documentation related to testing

Author: blcham

TESTING.md

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+# Testing Conventions
+
+This document outlines the testing conventions used in our project.
+
+## Test Class Naming
+
+- **Pattern**: `{ClassName}Test`
+- **Examples**:
+  - `ExecutionController` → `ExecutionControllerTest`
+  - `FormService` → `FormServiceTest`
+
+## Test Method Naming
+
+### For Service Methods
+- **Pattern**: `{methodName}{condition}{expected}`
+- **Examples**:
+  - `findUserByIdWhenUserExistsReturnsUser`
+  - `saveUserWhenEmailIsInvalidThrowsValidationException`
+
+## Test Structure (Given-When-Then)
+
+Always structure tests using the Given-When-Then pattern:
+
+### Basic Structure
+```java
+@Test
+@DisplayName("Descriptive test name only if method is too complex")
+void methodName_condition_expected() {
+    // Given - setup test data and preconditions
+    
+    // When - perform the action being tested
+    
+    // Then - verify the results
+}
+```
+
+### Example: Service Test (GWT parts are delimited by newline)
+```java
+@Test
+void findUserByIdWhenUserExistsReturnsUser() {
+    User expectedUser = new User("1", "test@example.com");
+    when(userRepository.findById("1")).thenReturn(Optional.of(expectedUser));
+    
+    User result = userService.findUserById("1");
+    
+    assertThat(result).isEqualTo(expectedUser);
+    verify(userRepository).findById("1");
+}
+```
+
+### Example: One-liner that is too simple to split into GWT parts
+```java
+@Test
+void findById_returnsEmpty_whenNotFound() {
+    assertThat(repository.findById("nonexistent")).isEmpty();
+}
+```
+
+## Best Practices
+
+1. **One Assertion Per Test**: Each test should verify one behavior
+2. **Descriptive Names**: Test names should clearly describe the scenario
+3. **Test Isolation**: Each test should be independent of others
+4. **Readability**: Use empty lines to separate GWT sections for better readability
+5. **Use @DisplayName**: For more descriptive test output only if it is hard to understand from the method name
+6. **Nested Tests**: Use `@Nested` to group related test cases