Add a way to set global properties in the JUnit output

[sunshowers]

• Define a way to set arbitrary global properties. For example, a NEXTEST_JUNIT_PROPERTY_foo = bar environment variable that becomes a property foo = bar.
• Make the root element a <testsuite>, not a <testsuites>. Nest <testsuite> elements for each binary within the parent <testsuite>. This is allowed by the XSD https://llg.cubic.org/docs/junit/ but some parsers might not like it so we might need to be careful.

[sunshowers]

Unfortunately the nested <testsuite> approach might not work because most tooling can't handle it -- they expect the <testsuites>/<testsuite>/<testcase> pattern. For example, see sebastianbergmann/phpunit#2964.

So the options are:

• Duplicate the global <properties> across all <testsuite> instances.
• Add <properties> to the global <testsuites>, going against the XSD.
• Put all test cases across all binaries within a single large <testsuite>.