Add custom SystemModules to bootclasspath to reduce duplication

Adds a library dependency to each of the dummy system modules created
by testing.go so that any changes in the behavior were detected by
the existing tests which were then fixed.

Bug: 141359858
Test: m checkbuild
Change-Id: Id4442f4aa3931ac93049f3367b96a5b49cc075e1

java/sdk_test.go

@@ -124,7 +124,7 @@ func TestClasspath(t *testing.T) {
 			name:          "nostdlib system_modules",
 			properties:    `sdk_version: "none", system_modules: "core-platform-api-stubs-system-modules"`,
 			system:        "core-platform-api-stubs-system-modules",
-			bootclasspath: []string{`""`},
+			bootclasspath: []string{"core-platform-api-stubs-system-modules-lib"},
 			classpath:     []string{},
 		},
 		{