testExplorer/go1.18: support running multiple subtests

[firelizzard18]

CL 343883 has been merged and presumably will be available in Go 1.18. When the user is using Go 1.18, this should allow us to run multiple subtests simultaneously.

• Given -run 'A/B|C/D'
• Go ≤ 1.17 interprets this as "Match test A → subtest B|C → subtest D"
• Go ≥ 1.18 should interpret this as "Match test A → subtest B or test C → subtest D"

Now that the precedence (at the top level) of alternation | is higher than the precedence of the test/subtest separator /, it is easy to construct a regex that will strictly match the requested tests and subtests (once 1.18 is released).