test_runner: add --test-coverage flag #7104

Job	Run time
Build slim tarball	1m 5s
aarch64-linux: Cache V8 build	21s
aarch64-darwin: with shared libraries	25m 31s
x86_64-darwin: with shared libraries	34m 32s
x86_64-linux: with shared libraries	26m 38s
aarch64-linux: with shared openssl-3.0.19	23m 53s
aarch64-linux: with shared openssl-3.6.1	23m 19s
aarch64-linux: with shared boringssl-0.20260413.0	22m 57s
aarch64-linux: with shared openssl-3.5.5	4s
aarch64-linux: with shared openssl-4.0.0	22m 43s
aarch64-linux: with shared openssl-1.1.1w	23m 10s
	3h 24m 13s

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