RFR: 8287828: Fix so that one can select jtreg test case by ID from make [v2]

[Leo Korinth]

> One can select a testcase by ID when running a jtreg test case directly from jtreg (using the testcase.java#testID syntax). However, this has not been possible to do when launching jtreg indirectly from make.
> 
> This fix attempts to address this issue. I have not tested this thoroughly yet, I wanted to show the code to get feedback first. The idea is to *not* emulated destructive imperative assignments through the use of eval. I instead try to handle it in a functional style without reassigning variables. I have tried to make the change as small as possible.
> 
> I am not used to change the build system, so I would appreciate thorough review.
> 
> `IfAppend` and `IfPrepend` are general purpose functions. If similar functions exists elsewhere inside the code base or in make itself I should probably use those instead. If they do not exist, they might be useful elsewhere and maybe should be placed in a common make file instead of the RunTests.gmk file.

Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:

  wip

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/9028/files
  - new: https://git.openjdk.java.net/jdk/pull/9028/files/553ecc35..704158c8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=9028&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=9028&range=00-01

  Stats: 82 lines in 2 files changed: 35 ins; 31 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9028.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9028/head:pull/9028

PR: https://git.openjdk.java.net/jdk/pull/9028