test-lib-functions: improve diagnostic output for trace2 data assertions

test_trace2_data is a bare grep that silently exits on failure.
Add a more informative variant that verifies the event appears
exactly once and reports what went wrong: key not found, multiple
entries, or value mismatch. Diagnostics go to FD 4 like test_grep.

Before (value mismatch):

  $ test_trace2_data status count/changed 999 <trace2.txt
  $ echo $?
  1
  (no output)

After:

  $ test_trace2_data_singular status count/changed 999 <trace2.txt
  error: trace2 data 'status/count/changed'
    expected: 999
    actual:   0

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions