Rewrite groovy tests as junit

.github/CONTRIBUTING.md

@@ -98,8 +98,8 @@ Once you are satisfied with your work, there is one last step to complete before
 
 ### <a name="#gh-style-guidelines"></a>Follow these Style Guidelines
 
-#### Java, Groovy, and Gradle
-We are using the [Google Java Style Guide](https://google.github.io/styleguide/javaguide.html) for Java, Groovy, and Gradle, with these exceptions and augmentations:
+#### Java and Gradle
+We are using the [Google Java Style Guide](https://google.github.io/styleguide/javaguide.html) for Java and Gradle, with these exceptions and augmentations:
 
 1. The recommended line width is 120, not 100.
    Modern screens are much wider than tall, so having wider lines allows more code to fit on a screen.

cdm/core/build.gradle

@@ -6,8 +6,6 @@ apply from: "$rootDir/gradle/any/dependencies.gradle"
 apply from: "$rootDir/gradle/any/java-library.gradle"
 apply from: "$rootDir/gradle/any/protobuf.gradle"
 
-apply plugin: 'groovy'  // For Spock tests.
-
 dependencies {
   api enforcedPlatform(project(':netcdf-java-platform'))
   implementation 'commons-math:commons-math'
@@ -32,9 +30,7 @@ dependencies {
   testImplementation 'com.google.truth:truth'
   testImplementation 'commons-io:commons-io'
   testImplementation 'junit:junit'
-  testImplementation 'org.codehaus.groovy:groovy-all' // for Spock.
   testImplementation 'org.mockito:mockito-core'
-  testImplementation 'org.spockframework:spock-core'
 
   testRuntimeOnly 'ch.qos.logback:logback-classic'
 }