doc: TestingOnARamDisk.md: Renamed env var for test dir to fix issue with internal build tool.
doc: TestingOnARamDisk.md: Renamed env var for test dir to fix issue with internal build tool.

Renamed TEST_TMPDIR to VT_TEST_TMPDIR because "TEST_TMPDIR" is a special environment variable recognized by our internal build tools.

If the variable is set, it prevents caching state of the build tool between consecutive runs. Therefore, we mustn't set it and use a different name.
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