Simplify Gradle configuration of CodegenTest

```
* Where:
Build file '/Users/lauzmata/aws-sdk-kotlin/tests/codegen/checksums/build.gradle.kts' line: 14

* What went wrong:
Script compilation errors:

  Line 14:     this@Build_gradle.tests.forEach { test ->
                   ^ Unresolved label.

  Line 14:     this@Build_gradle.tests.forEach { test ->
                                                 ^ Cannot infer type for this parameter. Specify it explicitly.

  Line 15:         projections.register(test.name) {
                                             ^ Unresolved reference 'name'.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                          ^ Cannot infer type for this parameter. Specify it explicitly.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                          ^ Function invocation 'model(...)' expected.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                                ^ Unresolved reference 'path'.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                                            ^ Cannot infer type for this parameter. Specify it explicitly.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                                            ^ Function invocation 'model(...)' expected.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                                                  ^ Unresolved reference 'fileName'.

  Line 16:             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
                                                                                                            ^ Unresolved reference 'asFile'.

  Line 18:                 serviceShapeId = test.serviceShapeId
                                                 ^ Unresolved reference 'serviceShapeId'.

  Line 19:                 packageName = "aws.sdk.kotlin.test.${test.name.lowercase()}"
                                                                     ^ Unresolved reference 'name'.
```

Author: lauzadis

tests/codegen/checksums/build.gradle.kts

@@ -6,12 +6,10 @@ import aws.sdk.kotlin.tests.codegen.Model
 
 description = "AWS SDK for Kotlin's checksums codegen test suite"
 
-val tests = listOf(
-    CodegenTest("checksums", Model("checksums.smithy"), "aws.sdk.kotlin.test#TestService"),
-)
-
 smithyBuild {
-    this@Build_gradle.tests.forEach { test ->
+    listOf(
+        CodegenTest("checksums", Model("checksums.smithy"), "aws.sdk.kotlin.test#TestService"),
+    ).forEach { test ->
         projections.register(test.name) {
             imports = listOf(layout.projectDirectory.file(test.model.path + test.model.fileName).asFile.absolutePath)
             smithyKotlinPlugin {