trapme: Xvfb startup timeout not obvious in logs

This is something I'm sure I've hit before (or certainly similar problems) but I don't think we've addressed it, so I'm raising an issue so it doesn't get lost. I've added `trapme` to the title so I can track any improvements to error conditions.

In https://ci.adoptium.net/job/Test_openjdk21_hs_sanity.openjdk_riscv64_linux/108/console we hit an issue where, for no clear reason, the tests didn't seem to start running. @sophia-guo spotted that it might be an issue with the Xvfb startup. Here is the log from a failing run:
```
8:54:41 ITERATION: 1/1
[Pipeline] wrap
18:54:41 $ Xvfb -displayfd 2 -screen 0 1024x768x24 -fbdir /home/jenkins/workspace/Test_openjdk21_hs_sanity.openjdk_riscv64_linux/.xvfb-108-..fbdir4921422941276978355
[Pipeline] // wrap
[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Post)
[Pipeline] step
18:55:21 TAP Reports Processing: START
18:55:21 Looking for TAP results report in workspace using pattern: aqa-tests/TKG/**/*.tap
18:55:21 Did not find any matching files. Setting build result to FAILURE.
```
And this is from a good run of the same thing on another machine:
```
10:04:20 ITERATION: 1/1
[Pipeline] wrap
10:04:20 $ Xvfb -displayfd 2 -screen 0 1024x768x24 -fbdir /home/jenkins/workspace/Test_openjdk21_hs_sanity.openjdk_riscv64_linux/.xvfb-115-..fbdir4816611008607352758
[Pipeline] {
[Pipeline] sh
10:04:32 + ps -f
10:04:32 + awk {print $9}
10:04:32 + grep [X]vfb
[Pipeline] echo
10:04:32 env.DISPLAY is :0
```
Ideally we should have something showing in the log to indicate that a timeout occurred (if indeed it can be confirmed that it is the cause of the failure)

Start by reviewing the linked Jenkins failure and the trapme/Xvfb startup path to determine how the timeout is handled. Done means a confirmed Xvfb startup timeout is clearly reported in the build log instead of only appearing as missing TAP results.

Written by the indexing model from the issue text.