tests: explicitly use `test-tool.exe` on Windows

tests: explicitly use `test-tool.exe` on Windows

In 8abfdf44c882 (tests: explicitly use `git.exe` on Windows,
2018-11-14), we made sure to use the `.exe` file extension when
using an absolute path to `git.exe`, to avoid getting confused with a
file or directory in the same place that lacks said file extension.

For the same reason, we need to handle test-tool.exe the same way.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>