tests: account for different newline behavior between Solaris and GNU grep

GNU grep, when emitting a matching line that doesn't have a terminating
newline, will add an extra newline.  Solaris grep passes the original line
through without the newline.  This causes differences in test output when
looking at the last line of the output of get-with-headers.py, which
doesn't usually emit (and certainly doesn't guarantee) a terminating
newline.

Both grep implementations succeed in matching the requested pattern,
though, so rely on specifying the full pattern on grep's commandline
instead of expecting it in the output, and send the output to /dev/null.

tests/test-http-protocol.t

@@ -16,11 +16,9 b''
 compression formats are advertised in compression capability
 
 #if zstd
-  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
-  compression=zstd,zlib
+  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep '^compression=zstd,zlib$' > /dev/null
 #else
-  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
-  compression=zlib
+  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep '^compression=zlib$' > /dev/null
 #endif
 
   $ killdaemons.py
@@ -29,8 +27,7 b' server.compressionengines can replace en'
 
   $ hg --config server.compressionengines=none -R server serve -p $HGPORT -d --pid-file hg.pid
   $ cat hg.pid > $DAEMON_PIDS
-  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
-  compression=none
+  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep '^compression=none$' > /dev/null
 
   $ killdaemons.py
 
@@ -38,8 +35,7 b' Order of engines can also change'
 
   $ hg --config server.compressionengines=none,zlib -R server serve -p $HGPORT -d --pid-file hg.pid
   $ cat hg.pid > $DAEMON_PIDS
-  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
-  compression=none,zlib
+  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep '^compression=none,zlib$' > /dev/null
 
   $ killdaemons.py