The --env/--boot flag has the same silent-bootstrap failure as #1179

[Chemaclass]

Problem

#1180 fixed the BASHUNIT_BOOTSTRAP env path. The --env / --boot flag
sources its file at different call sites, which still had the hole:

$ bashunit --no-parallel --env boot.sh t_test.sh
boot.sh: line 1: syntax error near unexpected token `{'
$ echo $?
0

No tests, no summary, exit 0. bench --boot behaves the same way.

The flag path validates that the file exists (#875) but not that sourcing it
succeeded — and it cannot, inline: source is a POSIX special builtin, so a
syntax error or a bare exit ends the shell at that point.

Fix

Apply the marker from #1180 to the remaining call sites — src/main/test.sh,
src/main/bench.sh, src/main/subcommands.sh — so the EXIT trap reports a
load that never finished.

Audited afterwards: 5 of 5 source "$boot_file" / source "$BASHUNIT_BOOTSTRAP" sites now carry the marker.

[Chemaclass]

Done in #1182 (merged).

[Chemaclass]

Correction: I stated the wrong mechanism

I wrote that source is a POSIX special builtin and that a syntax error in the
sourced file therefore terminates the shell. That is not what happens.
Measured:

case	result
set +e, syntax error	source returns 1, execution continues
set -e, syntax error	shell exits (status 2)
set +e, bare exit 3	shell exits (status 3)

So the two failure modes have different causes:

• Syntax error — source returns non-zero like any other command. The shell
  only dies because set -e is active at that point in the parse. Nothing to do
  with special builtins.
• Bare exit — that exits the shell whatever set -e says, because it is
  what exit means in a sourced file.

What this does and does not change

The fix stands: a marker plus the EXIT trap is still the design that covers
both, and it is still fork-free. But the alternative I dismissed —
set +e; source …; status=$? — would in fact have caught the syntax-error case;
it just would not have caught a bare exit. My reasoning for rejecting it was
wrong even though the conclusion held.

Recording this because a future reader would otherwise take the special-builtin
claim as fact.