testcover only works with Test::Harness, not TAP::Harness

[mephinet]

The action testcover needs to set -MDevel::Cover when tests are executed, so that coverage metrics are collected. Currently, it tries to achieve this by these lines of code:
(https://github.com/Perl-Toolchain-Gang/Module-Build/blob/master/lib/Module/Build/Base.pm#L2805)

local $Test::Harness::switches    =
local $Test::Harness::Switches    =
local $ENV{HARNESS_PERL_SWITCHES} = "-MDevel::Cover";

This is broken in two ways:

• Test::Harness::switches will be overwritten in run_test_harness, so setting it here has no effect whatsoever.
• HARNESS_PERL_SWITCHES is only valued by Test::Harness, but not TAP::Harness.

The result is that testcover only works with Test::Harness:

$ rm -rf cover_db; ./Build testcover
...
Result: PASS
HTML output written to .../cover_db/coverage.html

but not in TAP::Harness:

$ rm -rf cover_db; ./Build testcover use_tap_harness=1
...
Result: PASS
Can't open database .../cover_db

IMHO, the correct way for testcover to work would be to add -MDevel::Cover to the harness_switches, as they are passed on correctly to both Test::Harness and TAP::Harness.

Seen on Gentoo Linux amd64, Perl 5.20.2 and Module::Build from git master, as well as 0.4214.