Parameterise DataLoaderTest on DataLoader

The existing `DataLoaderTest` covers a very large range of test cases
which will be very useful to validate what is being added in graphql-java#148.

To allow new `*Publisher` `DataLoader`s to leverage this without
copy-pasting, we make `DataLoaderTest` a parameterised test, using two
`DataLoader` variants:

 - the stock 'List' `DataLoader`.
 - the 'Mapped' `DataLoader`.

Most of the tests in `DataLoaderTest` have been retrofitted for this
parameterisation, resulting in:

 - deduplicated code (many of the `MappedDataLoader` tests have the same
   test cases, almost line-for-line).
 - increased coverage of the `MappedDataLoader`, bolstering confidence
   in subsequent changes (rather than relying on the author
   understanding how everything is put together internally).

Author: AlexandreCarlton

build.gradle

@@ -72,6 +72,7 @@ dependencies {
     testImplementation 'com.github.ben-manes.caffeine:caffeine:2.9.0'
     testImplementation platform('org.junit:junit-bom:5.10.2')
     testImplementation 'org.junit.jupiter:junit-jupiter-api'
+    testImplementation 'org.junit.jupiter:junit-jupiter-params'
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine'
 }