Tests fail on Void Linux x86_64-musl

[Minoru]

Reported by package maintainers in Void Linux.

Newsboat version (copy the output of newsboat -v or the first line of git show): 2.21

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test is a Catch v2.13.1 host application.
Run with -? for options

-------------------------------------------------------------------------------
=~ and !~ use POSIX extended regex syntax
  No support for hexadecimal escape
-------------------------------------------------------------------------------
test/matcher.cpp:709
...............................................................................

test/matcher.cpp:713: FAILED:
  REQUIRE_FALSE( m.matches(&mock) )
with expansion:
  !true

-------------------------------------------------------------------------------
=~ and !~ use POSIX extended regex syntax
  No support for \a as alert/bell control character
-------------------------------------------------------------------------------
test/matcher.cpp:719
...............................................................................

test/matcher.cpp:723: FAILED:
  REQUIRE_FALSE( m.matches(&mock) )
with expansion:
  !true

-------------------------------------------------------------------------------
=~ and !~ use POSIX extended regex syntax
  No support for \b as backspace control character
-------------------------------------------------------------------------------
test/matcher.cpp:729
...............................................................................

test/matcher.cpp:733: FAILED:
  REQUIRE_FALSE( m.matches(&mock) )
with expansion:
  !true

-------------------------------------------------------------------------------
RegexManager uses POSIX extended regex syntax
  No support for hexadecimal escape
-------------------------------------------------------------------------------
test/regexmanager.cpp:570
...............................................................................

test/regexmanager.cpp:575: FAILED:
  REQUIRE( input == "value" )
with expansion:
  "<0>value</>" == "value"

-------------------------------------------------------------------------------
RegexManager uses POSIX extended regex syntax
  No support for \a as alert/bell control character
-------------------------------------------------------------------------------
test/regexmanager.cpp:578
...............................................................................

test/regexmanager.cpp:583: FAILED:
  REQUIRE( input == "\x07" )
with expansion:
  "<0>�</>" == "�"

-------------------------------------------------------------------------------
W3CDTF parser returns empty string on invalid input
-------------------------------------------------------------------------------
test/rsspp_rssparser.cpp:52
...............................................................................

test/rsspp_rssparser.cpp:56: FAILED:
  REQUIRE( rsspp::RssParser::w3cdtf_to_rfc822("-3") == "" )
with expansion:
  "Wed, 01 Jan -003 00:00:00 +0000" == ""

hello
===============================================================================
test cases:   495 |   492 passed | 3 failed
assertions: 22478 | 22472 passed | 6 failed