RFR: 8373113: Fix whitespace in RunTests.gmk

[Erik Joelsson]

While reading RunTests.gmk recently I found several instances of bad whitespace usage, tabs vs spaces. Correct usage of tabs is important in makefiles and even if none of these are particularly severe on their own, they can compound with other mistakes to create very hard to debug problems.

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Commit messages:
 - JDK-8373113

Changes: https://git.openjdk.org/jdk/pull/28668/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28668&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373113
  Stats: 9 lines in 1 file changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/28668.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28668/head:pull/28668

PR: https://git.openjdk.org/jdk/pull/28668