RFR: 8297071: Provide gradle "TEST_ONLY" flag to completely suppress building the sdk and shims

[Ambarish Rapte]

Add `TEST_ONLY` flag to gradle with default value as false.
Add new task named `shims`, This task depends on all shim tasks from all projects. Hence it will build all shim classes.

If TEST_ONLY is set to true, the all non test tasks get disabled. When running a test task, this flag would reduce time in checking if all non test tasks are up to date.
How to use:
1. Following command would fail in a clean repo, 
  > gradle -PTEST_ONLY=true :base:test
2. The sdk and shims task must have been executed before running any test
  > gradle sdk shims
  > gradle -PTEST_ONLY=true :base:test
3. Setting this flag to true with non test task would result in no task being run.
  > gradle -PTEST_ONLY=true sdk  -> It will not build sdk task

TIP: use `--dry-run` option of gradle for speeding up testing the effect of this PR.

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Commit messages:
 - test tasks depends on shims instead of sdk
 - Empty commit
 - all.dependsOn(shims)
 - Add TEST_ONLY flag to gradle

Changes: https://git.openjdk.org/jfx/pull/1117/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1117&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297071
  Stats: 41 lines in 2 files changed: 31 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1117.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1117/head:pull/1117

PR: https://git.openjdk.org/jfx/pull/1117