acceptance: de-flake test_missing_log_output

...by filtering out all GRPC log messages. The specific offender is
https://github.com/grpc/grpc-go/blob/v1.7.0/clientconn.go#L937; this is
the same error message that we throttle in the grpcutil log bridge. It
is a warning that reads:

> grpc: addrConn.resetTransport failed to create client transport: ...

Turns out there are other offenders, so we just filter out any log messages
that originates from GRPC. Another example is logged from
https://github.com/grpc/grpc-go/blob/v1.7.0/clientconn.go#L696 and reads:

> Failed to dial ...: ...; please retry.

Fixes #19397.
Fixes #19388.

pkg/cli/interactive_tests/test_missing_log_output.tcl

@@ -76,7 +76,7 @@ start_test "Test that quit does not emit unwanted logging output"
 # Unwanted: between the point the command starts until it
 # either prints the final ok message or fails with some error
 # (e.g. due to no definite answer from the server).
-send "echo marker; $argv quit 2>&1 | grep '^\\(\[IWEF\]\[0-9\]\\)' \r"
+send "echo marker; $argv quit 2>&1 | grep -vE '^\[IWEF\]\[0-9\]+ .\ vendor/google.golang.org/grpc/' | grep -vE '^\(ok|Error\)'\r"
 set timeout 20
 eexpect "marker\r\n:/# "
 set timeout 5
@@ -87,11 +87,7 @@ start_test "Test that quit does not show INFO by defaults with --logtostderr"
 # that the default logging level is WARNING, so that no INFO messages
 # are printed between the marker and the (first line) error message
 # from quit. Quit will error out because the server is already stopped.
-#
-# Since quit is likely to actually emit warnings, but we are just
-# interested in the absence of INFO messages, remove the warnings from
-# the output.
-send "echo marker; $argv quit --logtostderr 2>&1 | grep -v '^W'\r"
+send "echo marker; $argv quit --logtostderr 2>&1 | grep -vE '^\[WEF\]\[0-9\]+ .\* vendor/google.golang.org/grpc/'\r"
 eexpect "marker\r\nError"
 eexpect ":/# "
 end_test