t/test_lib: avoid naked bash arrays in file_lineno

t/test_lib: avoid naked bash arrays in file_lineno

662f9cf154 (tests: when run in Bash, annotate test failures with file
name/line number, 2020-04-11), introduces a way to report the location
(file:lineno) of a failed test case by traversing the bash callstack.

The implementation requires bash and uses shell arrays and is therefore
protected by a guard but NetBSD sh will still have to parse the function
and therefore will result in:

  ** t0000-basic.sh ***
  ./test-lib.sh: 681: Syntax error: Bad substitution

Enclose the bash specific code inside an eval to avoid parsing errors in
the same way than 5826b7b595 (test-lib: check Bash version for '-x'
without using shell arrays, 2019-01-03)

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>