RFR: 8349781: make test TEST=gtest fails on WSL

[Daniel Jeliński]

`make test TEST=gtest` fails to run gtest on WSL. This is because we're looking for a nonexistent file `gtestLauncher`; on Windows the file is named `gtestLauncher.exe`.

This PR adds `$EXECUTABLE_SUFFIX` to the executable path, similar to what [GtestImage.gmk already does](https://github.com/openjdk/jdk/blob/64bd8d2592d26e02a7f2f96caa47cba5e158aaa2/make/hotspot/test/GtestImage.gmk#L37).

I verified that with the proposed change, `make test TEST=gtest` works correctly both on WSL and on Cygwin. Other platforms are unaffected, because EXECUTABLE_SUFFIX is [empty on all non-Windows platforms](https://github.com/openjdk/jdk/blob/64bd8d2592d26e02a7f2f96caa47cba5e158aaa2/make/autoconf/toolchain.m4#L188).

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Commit messages:
 - Add exe suffix to gtestLauncher search path

Changes: https://git.openjdk.org/jdk/pull/23555/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23555&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349781
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23555/head:pull/23555

PR: https://git.openjdk.org/jdk/pull/23555