th:  Discard error output from tests

We don't process a test's stderr in th, and it only mucks up the output,
so suppress it.  If a test fails, running it directly produces the error
output.  (This is no worse than suppressing TAP comments to stdout.)
tap-out:  Rename ok_if() to print()

And add back ok_if() as an alias.
rectangle-math.mac:  Factor out LINEREF()

Document the brittleness of our use of __FILE__.  Fortunately, it's only
temporary.
rectangle-math.mac:  Call tap::print() for passes
rectangle-math.mac:  Negate the logic in expect()
rectangle-math.mac:  Write lineref to stderr

Write the file and line information to stderr rather than printing it in
the test case output as the TAP comment.

Under th, it's not displayed anyway, so this is no change.  We also gain
the option of redirecting stderr to a tool for displaying test failures,
in which case it's simpler than parsing the TAP output.  Finally, by not
relying on the TAP comment feature we can eventually remove it entirely.
rectangle-math.mac:  Use cc-style test diagnostics

We expect File and Line commands in the diagnostic display of MPW-hosted
tools, and a program like mwcc which knows it interacts with them can be
prepared to deal with such.  Diagnostic-printing tools that run directly
in MacRelix (a Unix-like environment) should use cc-style message lines,
since that's what other Unix tools expect.
Merge branch 'expect-rewrite'