RFR(XS) 8238198 Avoid using @ tags in TestOptionsWithRanges_generate.sh

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8238198
http://cr.openjdk.java.net/~iklam/jdk15/8238198-avoid-at-char-in-TestOptionsWithRanges_generate.sh.v01/

Hi, here's a better fix than what I did in JDK-8237046 [1]

It is probably a bug in jtreg that it thinks the script is a test due to
the existence of /* @test ... */. But to be safe, we should replace
@test with ${at}test.

Thanks
- Ioi

[1] 
https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-January/037899.html