testscript: remove our code coverage mechanism thanks to Go 1.20

gotooltest/testdata/cover.txt

@@ -1,7 +1,5 @@
 unquote scripts/exec.txt
 
-[darwin] skip 'Pending a fix for github.com/rogpeppe/go-internal/issues/130'
-
 # The module uses testscript itself.
 # Use the checked out module, based on where the test binary ran.
 go mod edit -replace=github.com/rogpeppe/go-internal=${GOINTERNAL_MODULE}
@@ -10,18 +8,17 @@ go mod tidy
 # First, a 'go test' run without coverage.
 go test -vet=off
 stdout 'PASS'
-! stdout 'total coverage'
+! stdout 'coverage'
 
 # Then, a 'go test' run with -coverprofile.
-# Assuming testscript works well, this results in the basic coverage being 0%,
-# since the test binary does not directly run any non-test code.
-# The total coverage after merging profiles should end up being 100%,
-# as long as all three sub-profiles are accounted for.
+# The total coverage after merging profiles should end up being 100%.
 # Marking all printlns as covered requires all edge cases to work well.
+# Go 1.20 learned to produce and merge multiple coverage profiles,
+# so versions before then report a shallow 0% coverage.
 go test -vet=off -coverprofile=cover.out -v
 stdout 'PASS'
-stdout '^coverage: 0\.0%'
-stdout '^total coverage: 100\.0%'
+[go1.20] stdout 'coverage: 100\.0%'
+[!go1.20] stdout 'coverage: 0\.0%'
 ! stdout 'malformed coverage' # written by "go test" if cover.out is invalid
 exists cover.out