[Test][Time profiler] Fix test time checking

This test sometimes triggers failures during build testing. For instance, see:
https://lab.llvm.org/buildbot/#/builders/52/builds/10161, details: https://lab.llvm.org/buildbot/#/builders/52/builds/10161/steps/5/logs/FAIL__Clang__check-time-trace-sections_cpp .
AFAICT the time between driver calling and checking its time tracker output
is not guaranteed to be stable and small:
```
> head -2 check-time-trace-sections.cpp
// RUN: %clangxx -S -ftime-trace -ftime-trace-granularity=0 -o %T/check-time-trace-sections %s
// RUN: cat %T/check-time-trace-sections.json | %python %S/check-time-trace-sections.py
> clang -S -ftime-trace -ftime-trace-granularity=0 -o /tmp/check check-time-trace-sections.cpp
> cat /tmp/check.json | python check-time-trace-sections.py
> sleep 10
> cat /tmp/check.json | python check-time-trace-sections.py
'beginningOfTime' should represent the absolute time when the process has started
>
```
The attribute `beginningOfTime` was introduced here: https://reviews.llvm.org/D78030
One can change "10 sec" value to something longer, but I believe
it's enough just to check that `beginningOfTime` exists and is
not later than current time.

Differential Revision: https://reviews.llvm.org/D108625