RFR: 8352982: gradle TEST_SDK_PATH param doesn't work with relative paths

[Ambarish Rapte]

Issue:
The test execution fails when a relative path is specified a relative path for `TEST_SDK_PATH`.
For example:
1. gradle -PTEST_ONLY=true -PTEST_SDK_PATH=**..**/jfx1/build test 
2. gradle -PTEST_ONLY=true -PTEST_SDK_PATH=**~**/jfx1/build test

Solution:
Convert the relative path to an absolute path.

More about fix:
The property TEST_SDK_PATH belongs to the root project rt in build.gradle.
If we modify this property in build.gradle, it does not reflect in the child projects. The child projects for example graphics, controls would still have the value of TEST_SDK_PATH before modification.
To solve this, I Introduced a new variable in build.gradle `TEST_SDK_DIR`. It would hold the modified value of `TEST_SDK_PATH` and gets correctly reflected across all sub-projects.

Verified that both relative paths and absolute path work fine after this change.

-------------

Commit messages:
 - relative test sdk path

Changes: https://git.openjdk.org/jfx/pull/1751/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1751&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352982
  Stats: 28 lines in 1 file changed: 12 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jfx/pull/1751.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1751/head:pull/1751

PR: https://git.openjdk.org/jfx/pull/1751